Add 'php_conf_mysql_default_socket' variable to lxc-php role that
configure both the lxc containers and PHP so that a local MySQL database
may be used through localhost.
The PHP containers will automount /var/run/mysqld/mysqld.sock (the
default path to the mysql socket) to the path defined by the variable
'php_conf_mysql_default_socket' which will be the path used by php to
contact MySQL both with mysqli and PDO_MYSQL.
