Fix retyping attached volumes requiring migration

Fix retyping attached volumes requiring migration

Modifies retype method so that volume db entry only updated if the
volume isn't migrated.

The migrate_volume_completion method is made responsible for detaching
the old volume and attaching the new one. This takes the requirement
from nova because the calls need to be done in a certain order which
isn't guaranteed when performed by nova.

attach_volume now marks the migration_status as complete because it is
the final action by cinder.

Change-Id: Ia15636893be4f0077a4f75e746a8ab1a8798c44b
Partial-Bug: 1316079