Allow/document configuration of non-SSL Registry

First, big thanks to the GitLab team for implementing the registry functionality. It's awesome!

The registry admin documentation reads:

Note: The container Registry works under HTTPS by default. Using HTTP is possible but not recommended and out of the scope of this document. Read the insecure Registry documentation if you want to implement this.

The link is to the Docker engine/client documentation on connecting to insecure registries, not anything related to the registry itself.

As noted at #1309 (closed), #1300 (closed), #1304 (closed) there appears to either be no real support for running GitLab's registry non-encrypted (e.g., when using a load balancer or reverse proxy for SSL termination, or because you like danger) or there is just no documentation on the configuration of the Registry Nginx server.