Restores Nova API for volume attach and detach

Reasons:
 - Cinder's API attach-detach APIs do not actually work with
   Nova instances.
 - Cinder APIs only update metadata for attach/detach operations.

Changes:
 - Replace cinder calls for attaching and detaching volumes
   with nova API calls.
 - Removed duplicate delete calls to prevent race condition.
 - Removed duplicate mocked calls from unit-test.

Closes-Bug: #1311533
Change-Id: I5f16c528652f12440160f03b92f41b76d1c9100c