mon: allow a MON_DOWN grace period after cluster mkfs

mon: allow a MON_DOWN grace period after cluster mkfs

During teuthology tests, the initial cluster bootstrap often starts up
the mon sbut doesn't include all mons in the initial quorum, due to
mon startup misalignment and random delays.  Provide a short grace period
where we will not raise a MON_DOWN alert even though the quorum is not
complete.

Fixes: https://tracker.ceph.com/issues/43584
Signed-off-by: Sage Weil <sage@newdream.net>