mon: prevent pool with snapshot state from being used as a tier

mon: prevent pool with snapshot state from being used as a tier

If we add a pool with snap state as a tier the snap state gets clobbered
by OSDMap::Incremental::propogate_snaps_to_tiers(), and may prevent OSDs
from starting.  Disallow this.

Include a test.

Fixes: #11493
Backport: hammer, giant, firefly
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit bbec53edf9e585af4e20bbc9ba9057d6fdfda342)

Conflicts:
	qa/workunits/cephtool/test.sh
           properly co-exist with "# make sure we can't create an ec pool tier"
	src/mon/OSDMonitor.cc
           properly co-exist with preceding "if (tp->ec_pool())"
        (The changes to both files would have applied cleanly if
        https://github.com/ceph/ceph/pull/5389 had not been merged first.)