can't delete (rbd) snapshot if volume missing

[Edward Hope-Morley]

If I try to delete a snapshot whose base volume is ImageNotFound in Ceph the operation currently fails with https://pastebin.ubuntu.com/p/CjK8FGJJtw/ because the RBDVolumeProxy does not tolerate a missing volume an we don't handle the exception like we would for a volume delete. If the base volume is missing this in all likelihood indicates an error on the backend and since a snapshot can't exist without its base, it is valid to allow the delete to succeed. We should therefore handle this case to allow snapshots to be deleted as well as volumes in the case where the volume and/or snapshot does not exist on the backend.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/658993