fix(controller): Determine registry address during start

Before this change re-deploying apps failed when the registry was re-scheduled to a different instance in the cluster.

Fixes #1128

- [x] Store just the image name and tag in the build model
- [x] Prefix the image reference before fleet pulls the image
- [x] Prefix the image reference before controller pushes config changes to the registry
- [x] Allow to migrate from existing releases
- [x] Don't block #1412
I added it to easily differentiate between images store somewhere, new images stored internally and old images stored internally. Maybe it would work without, but this would further complicate the logic and the existing sha 'hack' https://github.com/deis/deis/pull/1325/files#diff-211dbf56ab4ba80bf5d2b2a028b749daL479 will likely break with #1412 as this PR introduces images without a sha, which are internally stored.