[Feature] Move datadir after install (issue #25)

When ``node['mariadb']['mysqld']['datadir']`` is different than the default ``/var/lib/mysql``, this will trigger after install a script that stop the service, moves the datadir to its new location, replace it with a symlink to the new path and restart the service. Tested on Ubuntu 14.04 with MariaDB 10.0
I think that it's better to put the mv && ln in the command directly (without creating a script file)... As soon as the service is restarted in the precedent block, are you agree ?