Avoid mysql_user to be executed on every cluster node.

One node is enough since we are building a cluster. Choose $galera_master
for that.

See puppetlabs-mysql/manifests/server/root_password.pp for details.
As a side effect File["${::root_home}/.my.cnf"] is skipped, too, on
every node except $galera_master.  We have to change puppetlabs-mysql
to get around this.
Set $rhel_mysql_client_repo_hack on RHEL if using mariadb.

On RHEL we need an extra repository before installing
MariaDB-Galera-server, MariaDB-client, or galera.

This was tested on RHEL 6, only. Maybe RHEL 7 needs further modifications.
Try to improve behavior during "mass" deployments (>= 10 nodes).

Don't try to figure out, if a designated galera master is the only
node. Instead wait on every node != $galera_master if the master is
reachable.

"sleep 30" is quick and dirty to give some extra buffer.