mon/MgrMonitor: populate available_modules from promote_standby()
This was done in the beacon path, where there is no active mgr and we
get a new entrant, but not for this case where an existing standby is
promoted to active.
This fixes a problem during upgrade where a new (standby) mgr's modules
have a new module option but it is not reflected immediately (not until
the next beacon).
Fixes: https://tracker.ceph.com/issues/49778 Signed-off-by: Sage Weil <sage@newdream.net>