Bug #1937084 details a race condition within Cinder where requests to
delete an attachment and later delete the underlying volume can race
leading to the initial request returning a 404 if the volume delete
completes first.
This change attempts to handle this within Nova during a detach as we
ultimately don't care that the volume and/or volume attachment are no
longer available within Cinder. This allows Nova to complete its' own
cleanup of the BlockDeviceMapping record resulting in the volume no
longer appearing attached in Nova's APIs.
Closes-Bug: #1937084
Change-Id: I191552652d8ff5206abad7558c99bce27979dc84