Use pool/volume_name notation when deleting RBD volumes

the rbd command supports two methods for specifying a pool name:
the "-p" flag, and using the volume name notation "pool_name/volume_name"
The latter method supercedes the former, so to guard against slashes
in the volume name confusing things, always use the path notation

Change-Id: Ie79ae10b0fafefd85dea8e38cd24da0941b38bee
Closes-Bug: 1306285
Merge "Use pool/volume_name notation when deleting RBD volumes"