Add RBD as a known store

Adding RBD as a known store in the glance-api.conf file allows us to use
Ceph as a backend for Glance.

Closes-Bug: 1369578

Change-Id: I02cbafa68ca3293cedc9fef7535e79930cc4ee5c
Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge "Add RBD as a known store"