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.
Yep, I tried to be as close as possible to https://github.com/dotcloud/docker-registry/blob/master/config/config_sample.yml so future changes are easier to port.