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.
The search index is not activated when seach_backend is not set: https://github.com/dotcloud/docker-registry/blob/master/README.md#search-engine-options I think this should stay like this, until search is required to reduce complexity.