Instance and volume cleanup when a build fails

On failed builds the _shutdown_instance method used to get called which
would clean up leftover instance artifacts, volume attachments, and
networking.  This no longer happens which is causing volumes to be left
in an attached state when they're not attached to anything.

Network deallocation is already handled in this code path so it should
not happen in _shutdown_instance.

Change-Id: I899b64ac5941acc282ccae7e1963d6f714c01e8b
Closes-bug: #1332198
Merge "Instance and volume cleanup when a build fails"