Continue to delete volumes that DNE in rbd backend

If a volume has already been deleted in the RBD backend, catch the
exception and let Cinder continue removing it in the database.

Closes-Bug: #1254318
Change-Id: I99b591ce78d82c6eaabff0459d9c523e85c8e65f
(cherry picked from commit 3de7da12d1098ef777305099e5f4a039e536bf99)
Merge "Continue to delete volumes that DNE in rbd backend" into stable/havana