osd: restart boot process if waiting for luminous mons

osd: restart boot process if waiting for luminous mons

If we start_boot and see that we don't have luminous mons, we will stop.
But we don't currently reliably notice when the luminous upgrade completes.
If we happen to be connected to the last mon we will start_boot() because
of the trigger in ms_handle_connect(), but if we are not connected to the
last mon we'll eventually get a monmap update but not restart booting.

Fix by setting a flag if we are waiting, and restart boot if the flag is
set, we are in preboot, and we see we now have luminous mons.

Fixes: http://tracker.ceph.com/issues/20631
Signed-off-by: Sage Weil <sage@redhat.com>