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.
In that case, why shouldn't we just put the registry up in the list after `cache`? The list determines its starting order, so if you moved it after `cache` then the `/deis/cache` unit should be available at boot time.