Galera cluster maintenance

ops-galera-remove.rst ops-galera-start.rst ops-galera-recovery.rst ops-galera-recoverysingle.rst ops-galera-recoverymulti.rst ops-galera-recoverycomplete.rst ops-galera-recoverycontainer.rst

Routine maintenance includes gracefully adding or removing nodes from the cluster without impacting operation and also starting a cluster after gracefully shutting down all nodes.