俺です。

terraform v0.6.11で書いたやつです。
subnetは自分で作ってねという。
任意のwriter nodeとparameter groupを指定できないのが痛い。
ただwriter nodeのfailoverは40秒ぐらいで終わるので作った後ターゲットのAZでなければ手動failしてしまえばよいかなと。

resource "aws_rds_cluster_instance" "aurorasaikou" {
  cluster_identifier = "${aws_rds_cluster.aurorasaikou.id}"
  identifier         = "${aws_rds_cluster.aurorasaikou.cluster_identifier}-${count.index}"
  instance_class     = "db.r3.large"
  db_subnet_group_name = "${aws_db_subnet_group.backend.name}"
  publicly_accessible = false
  count = 3
  depends_on = ["aws_db_subnet_group.backend"]
}

resource "aws_rds_cluster" "aurorasaikou" {
  cluster_identifier = "aurorasaikou"
  availability_zones = ["ap-northeast-1a", "ap-northeast-1c"]
  database_name = "aurorasaikou"
  master_username = "aurorasaikouuser"
  master_password = "Aurorasa1ko%"
  final_snapshot_identifier = true
  port = 3306
  apply_immediately = true
  vpc_security_group_ids = [""${aws_security_group.default.id}"]
  db_subnet_group_name = "backend"
}

完