Image on private registries

[codekoala]

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.