(proposal): use registry-to-registry transfer

Note that this is a PR made against the `deis-build` branch.

Instead of running a docker engine in the controller, I implemented a new feature to the registry that allows a user to pull in a repository from one registry to another. This greatly simplifies the `deis-build` PR. Note that the public index does not work due to the endpoints being (mostly) closed source. This must be implemented upstream in https://github.com/deis/docker-registry/compare/repository-import.

The meat of this is hard to write tests for, but will get added eventually in the upstream PR before this is merged into master. I just needed this PR to work and get some discussion going :)

I've also contributed some fixes to the comments that I made in #1190