mgr: wait for ~3 beacons on startup if mons are pre-pacific

mgr: wait for ~3 beacons on startup if mons are pre-pacific

If we are going active and the mons are pre-pacific, they may have the
bug https://tracker.ceph.com/issues/49778 which prevents our modules
metadata (including options) from being updated (until the next beacon).
Wait a bit (6s by default, 3x the 2s mgr_tick_period) to let this
happen.

This allows us to upgrade from broken pre-pacific mons using cephadm,
which may (if orig cluster is <15.2.5) immediately do a cephadm
migration that relies on the mgr/cephadm/migration_current config
option being present in the mon's mgrmap.

Workaround for https://tracker.ceph.com/issues/49778

Signed-off-by: Sage Weil <sage@newdream.net>