Document Image Registry migration from Cinder to Swift

Provide a migration path to allow users to switch the Image Registry backend from a persistent volume to Swift. Swift allows for concurrent access, thus redundancy of image-registry, and is the recommended IR backend.

In case Swift becomes available in existing clusters, or in case the installation for incidental reasons failed over to Cinder, users must be able to start using Swift as a day-2 operation, while recovering images they may have pushed to the volume-backed registry up to that point.