Refact the mds cluster upgrade code in order to follow the documented
recommandation.
See: https://github.com/ceph/ceph/blob/luminous/doc/cephfs/upgrading.rst

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1569689



Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 71cebf80)