mgr/cephadm: use _apply_service for mgrs

mgr/cephadm: use _apply_service for mgrs

Note that we are losing some of the special logic about removing standby
mgrs only.  This should be added back *after* we fix up the scheduler
to be more intelligent about choosing hosts that already host daemons,
and make removal pick hosts that aren't selected (by label, or by
scheduler, etc.).

A few bugs to track this:
 https://tracker.ceph.com/issues/44167
 https://tracker.ceph.com/issues/44252 (prefer standby mgrs *and* mdss)

Signed-off-by: Sage Weil <sage@redhat.com>