Fix swap_volumes

The attach and detach calls to cinder are better handled by cinder this
patch removes them. There is another patch against this bug to modify
cinder.

The attach call is being made against the new volume after the
migrate_volume_completion request. This means it may be done after the
new volume ID in the db has been deleted.

The detach call is being run at the end on the old ID and so may end up
detaching the migrated volume from cinder's perspective.

Change-Id: I3f2d98e9e473905a3de2f02e00c92ae3065e6ae7
Partial-Bug: 1316079
Merge "Fix swap_volumes"