Ceph

Ah, there are Monitor checks for this but they aren't quite right: they look at the contents of snaps and removed_snaps, but don't consider the possibility that these things previously existed but have been erased.
I'm simplifying this by making use of the is_pool_snaps_mode() check (and adding a similar is_unmanaged_snaps_mode()).

Pushed a fix (and tested it using rados snap create and rbd snap create) to wip-2345-snaps.