Reverse order of cinder.detach() and bdm.delete()

The user experience for detaching a volume is better if the bdm
is destroyed as soon as the driver has detached the volume instead
of waiting until after we have updated cinder.

Change-Id: I6f3de25f7737db270af4ba41d7451ae95f3950f5
Closes-Bug: #1309154
Merge "Reverse order of cinder.detach() and bdm.delete()"