osd: PeeringState: fix stretch peering so PGs can go peered but not active

osd: PeeringState: fix stretch peering so PGs can go peered but not active

I misunderstood and there was a pretty serious error here: to prevent
accidents, choose_acting() was preventing PGs from *finishing* peering
if they didn't satisfy the stretch cluster rules. What we actually want
to do is to finish peering, but not go active.

Happily, this is easy to fix -- we just add a call to stretch_set_can_peer()
alongside existing min_size checks when we choose whether to go PG_STATE_ACTIVE
or PG_STATE_PEERED!

Signed-off-by: Greg Farnum <gfarnum@redhat.com>