When rbd is used for ephemeral disks and image storage, make snapshot
use ceph directly, and update glance with the new location. In
case of failure, it will gracefully fallback to the "generic"
snapshot method.
This requires changing the typical permissions for the nova ceph
user (if using authx) to allow writing to the pool where vm images
are stored, and it also requires configuring Glance to provide a
v2 endpoint with direct_url support enabled (there are security
implications to doing this).
Spec: Ia3666d53e663eacf8c65dbffbd4bc847dd948171
DocImpact: Ceph and Glance configuration changes
Implements: blueprint rbd-instance-snapshots
Change-Id: If13d25b6c94e247d2505583b524ae3af9c34b5dc