yumのデフォルトでは古いnginxがインストールされるので、新しくリポジトリファイルを作ってそちらを見に行くようにした

これは安定版

stableVersionの/etc/yum.repos.d/nginx.repo

[nginx]
name=nginx repo
baseurl=http://nginx.org/packages/centos/7/$basearch/
gpgcheck=0
enabled=1

今回最新版を入れたかったので、以下のように変更

/etc/yum.repos.d/nginx.repo

[nginx]
name=nginx repo
baseurl=http://nginx.org/packages/centos/7/$basearch/(これを)
↓
baseurl=http://nginx.org/packages/mainline/centos/7/$basearch/(こうする)
gpgcheck=0
enabled=1

一度removeしてからinstallし直す

$ sudo yum -y --enablerepo=nginx remove nginx

$ sudo yum -y --enablerepo=nginx install nginx
Loaded plugins: fastestmirror, langpacks
Loading mirror speeds from cached hostfile
 * base: ftp.iij.ad.jp
 * epel: mirror.dmmlabs.jp
 * extras: ftp.iij.ad.jp
 * remi-safe: repo1.sea.innoscale.net
 * updates: ftp.iij.ad.jp
Resolving Dependencies
--> Running transaction check
---> Package nginx.x86_64 1:1.12.2-1.el7_4.ngx will be installed
--> Finished Dependency Resolution

Dependencies Resolved

==================================================================================================================================
 Package                   Arch                       Version                                     Repository                 Size
==================================================================================================================================
Installing:
 nginx                     x86_64                     1:1.12.2-1.el7_4.ngx                        nginx                     716 k

Transaction Summary
==================================================================================================================================
Install  1 Package

Total download size: 716 k
Installed size: 2.5 M
Downloading packages:
nginx-1.12.2-1.el7_4.ngx.x86_6 FAILED                                          
http://nginx.org/packages/mainline/centos/7/x86_64/RPMS/nginx-1.12.2-1.el7_4.ngx.x86_64.rpm: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
To address this issue please refer to the below knowledge base article 

https://access.redhat.com/articles/1320623

If above article doesn't help to resolve this issue please create a bug on https://bugs.centos.org/
Error downloading packages:
  1:nginx-1.12.2-1.el7_4.ngx.x86_64: [Errno 256] No more mirrors to try.

1.13をインストールしたいのに、なぜか頭の方で
nginx.x86_64 1:1.12.2-1.el7_4.ngxをインストールすることが決まっている
nginx.repoを指定しているのだから、そのbaseurlを見に行くものかと思いきや
メッセージを見るとcached hostfileから決められているようだ
なのでその__cached hostfile__を消す
【yum】yumが動かないときの簡易対処方法集（IOError、Trying other mirror、Determining fastest mirrors、フリーズ等）
こちらを参考にしました。

$ sudo rm -f /var/run/yum.pid
$ sudo rm -rf /var/lib/rpm/__db.00?
$ sudo rpm --rebuilddb
$ sudo yum clean all

Loaded plugins: fastestmirror, langpacks
Cleaning repos: base epel extras nginx remi-safe updates
Cleaning up everything
Maybe you want: rm -rf /var/cache/yum, to also free up space taken by orphaned data from disabled or removed repos
Cleaning up list of fastest mirrors

と言われたので、

$ sudo rm -rf /var/cache/yum/x86_64/7/

こちらも削除。使わないキャッシュらしい。

もう一度installすると、キャッシュファイルが再構築され無事インストール出来ました。

$ sudo yum -y --enablerepo=nginx install nginx.x86_64
Loaded plugins: fastestmirror, langpacks
base                                                                                                       | 3.6 kB  00:00:00     
epel/x86_64/metalink                                                                                       | 5.9 kB  00:00:00     
epel                                                                                                       | 4.7 kB  00:00:00     
http://ftp.jaist.ac.jp/pub/Linux/Fedora/epel/7/x86_64/repodata/repomd.xml: [Errno -1] repomd.xml does not match metalink for epel
Trying other mirror.
epel                                                                                                       | 4.7 kB  00:00:00     
http://ftp.riken.jp/Linux/fedora/epel/7/x86_64/repodata/repomd.xml: [Errno -1] repomd.xml does not match metalink for epel
Trying other mirror.
epel                                                                                                       | 4.7 kB  00:00:00     
extras                                                                                                     | 3.4 kB  00:00:00     
nginx                                                                                                      | 2.9 kB  00:00:00     
remi-safe                                                                                                  | 2.9 kB  00:00:00     
updates                                                                                                    | 3.4 kB  00:00:00     
(1/9): base/7/x86_64/group_gz                                                                              | 156 kB  00:00:00     
(2/9): extras/7/x86_64/primary_db                                                                          | 130 kB  00:00:00     
(3/9): epel/x86_64/group_gz                                                                                | 261 kB  00:00:00     
(4/9): base/7/x86_64/primary_db                                                                            | 5.7 MB  00:00:00     
(5/9): updates/7/x86_64/primary_db                                                                         | 3.6 MB  00:00:00     
epel/x86_64/primary_db         FAILED                                          
http://ae.mirror.rasanegar.com/fedoraproject/pub/epel/7/x86_64/repodata/5e3e7ed1a51fc4f466daad159593b466163d334366d456b2e0f3e0403c6ae66d-primary.sqlite.bz2: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
To address this issue please refer to the below knowledge base article 

https://access.redhat.com/articles/1320623

If above article doesn't help to resolve this issue please create a bug on https://bugs.centos.org/

(6/9): nginx/x86_64/primary_db                                                                             |  87 kB  00:00:01     
(7/9): epel/x86_64/primary_db                                                                              | 6.1 MB  00:00:01     
(8/9): remi-safe/primary_db                                                                                | 1.1 MB  00:00:02     
epel/x86_64/updateinfo         FAILED                                          
http://mirror.dmmlabs.jp/linux/fedora/epel/7/x86_64/repodata/3c7c3e414b0217c165d60c62bf30315d663c266f8922f742544db9a3eaddc6b3-updateinfo.xml.bz2: [Errno 14] HTTP Error 404 - Not Found
Trying other mirror.
(9/9): epel/x86_64/updateinfo                                                                              | 847 kB  00:00:00     
Determining fastest mirrors
 * base: ftp.iij.ad.jp
 * epel: ftp.riken.jp
 * extras: ftp.iij.ad.jp
 * remi-safe: mirrors.thzhost.com
 * updates: ftp.iij.ad.jp
Resolving Dependencies
--> Running transaction check
---> Package nginx.x86_64 1:1.13.6-1.el7_4.ngx will be installed
--> Finished Dependency Resolution

Dependencies Resolved

==================================================================================================================================
 Package                   Arch                       Version                                     Repository                 Size
==================================================================================================================================
Installing:
 nginx                     x86_64                     1:1.13.6-1.el7_4.ngx                        nginx                     723 k

Transaction Summary
==================================================================================================================================
Install  1 Package

Total download size: 723 k
Installed size: 2.5 M
Downloading packages:
nginx-1.13.6-1.el7_4.ngx.x86_64.rpm                                                                        | 723 kB  00:00:02     
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
warning: Failed to open SELinux handle.
  Installing : 1:nginx-1.13.6-1.el7_4.ngx.x86_64                                                                              1/1 
----------------------------------------------------------------------

Thanks for using nginx!

Please find the official documentation for nginx here:
* http://nginx.org/en/docs/

Please subscribe to nginx-announce mailing list to get
the most important news about nginx:
* http://nginx.org/en/support.html

Commercial subscriptions for nginx are available on:
* http://nginx.com/products/

----------------------------------------------------------------------
  Verifying  : 1:nginx-1.13.6-1.el7_4.ngx.x86_64                                                                              1/1 

Installed:
  nginx.x86_64 1:1.13.6-1.el7_4.ngx                                                                                               

Complete!

もう一回再現してみると

sudo rm -rf /var/cache/yum/x86_64/7/

だけで出来たので、__/var/cache/yum/x86_64/7/nginx/の中の(英数字羅列)1-primary.sqlite.bz2__の中にそのリストみたいなのが入ってる気がするのですが、
中がバイナリで見方がわからない