It seems that once a backup has been created of a VM, ceph declines to delete the image when the VM is deleted due to associated snapshots.
- Add a cleanup step to our daily backup jobs (probably easiest)
- Stop having backups (T289282) (even easier but not necessarily the wisest)
- Stop persisting snapshots in between backup jobs (will probably increase backup storage considerably)
- Hook nova or something so that we synchronously delete backups and snaps as part of VM deletion (hardest and most 'correct')