Use root pass for galera init script

The galera init script uses the debian-sys-maint account to shutdown
mysql. However this password cannot be preseeded, and is randomly
generated on install. Also it is set in the database so must be updated
when replication has been achieved.

This patch implements an alternative: Use the mysql root password for
init script operations (shutdown). This password is preseedable and will
work at any state of replication.

This is no less secure than previous state as we already write root galera
credentials to /root/.my.cnf (0600) and /etc/mysql/debian.cnf is also 0600.

Related: #411