mon/MDSMonitor: enforce mds_join_fs cluster affinity

mon/MDSMonitor: enforce mds_join_fs cluster affinity

Before this, "mds_join_fs" config enforced a preference for the standby
the monitors would select. Now the monitors actively enforce this
by purposefully removing an MDS wither lower "affinity". An MDS standby
has highest affinity if its mds_join_fs is the file system in question
or a vanilla standby (no mds_join_fs).

Fixes: https://tracker.ceph.com/issues/43392
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>