On a running galera cluster, we don't want to unintentionally allow a
galera_cluster_name from changing. This will cause a cluster to fail to
start (if the nodes are restarted sequentially) as the cluter name value
has changed.
To ensure we can still change this value, a new
"galera_force_change_cluster_name" variable is added, which can be set
to "True" in order to change the cluster name.
Change-Id: I8bae038bd207a15d5731b9c91b78df851a4c1986
