feat(registry) Configurable storage backends

Update the docker-registry to 0.7.3 and refactored the configuration template to allow configuration of the
storage backend using etcd

This PR is build on top of #1183 and fixes #1119

I tested this using local data storage and with S3.
> I thought fleet start is non-blocking and doesn't wait for the previous units.

Right, that was the old behaviour of fleet, my bad.

Isn't it fine to wait for the cache to just come up though? It's still going to boot... It just has to wait until `/deis/cache` is populated and then it should start booting normally. I still don't see why removing the registry from `START_COMPONENTS` is necessary.