Ignore DiskNotFound error on detaching volumes

When volume_api.terminate_connection RPC calls fail due to timeout,
detaching volume will be impossible because it is already detached but
still have the volume mapping entry. To make it detachable completely
by retrying volume detach, this patch make it ignore DiskNotFound error
while detaching volumes and go through removing the mapping.

Change-Id: I95077179160a2eb35436db2a5f7a5fbca64ec323
Closes-Bug: #1367964
Merge "Ignore DiskNotFound error on detaching volumes"