Let galera_new_cluster use "restart" instead of "start"

The galera_new_cluster command uses "systemctl start" to start up a the initial node of a new galera cluster.

Unfortunately "systemctl start" does nothing when a service is already started, it just returns success right away. And so does galera_new_cluster then, even though it didn't actually create a new cluster at all.

So I suggest to change "systemctl start" in the script to "systemctl restart", to enforce a fresh mysqld process start even in cases where the mariadb process is already running.