Image on private registries

Building on top of my other pull request, this one addresses the use of images that reside in private registries.

Normally, the image will look like ``repo/image``. In the case of an image on a private registry, the domain/IP (and maybe port) is also included in the image name, looking more like ``domain.com/repo/image`` or ``1.2.3.4:5678/repo/image``.

When your docker-register project places information for these images into etcd, they go into a key that looks like ``/backends/repo/image/imgid``. This change will pull just the last two slash-separated bits of the key, handling both public and private images.