Centos7 / Mariadb fixes
Centos7 / Mariadb fixes

* Add mariadb-server package to the yum installation
* Add root password initial settings ("post"seed fashion)
* Handle centos "mysql" service name according to MariaDB switch
* add modoboa@localhost in order to give access
modoboa/modoboa-installer,modoboa/modoboa-installer