No package mysql-server available.

Centos 7 comes with MariaDB instead of MySQL. MariaDb is a open source equivalent to MySQL and can be installed with yum -y install mariadb-server mariadb. If you must have mysql you need to add the mysql-community repo sudo rpm -Uvh http://dev.mysql.com/get/mysql-community-release-el7-5.noarch.rpm and then you can install MySQLl like you normally do.