Rely on IP address for metadata server

The client code uses the hard-coded IP and "metadata" to contact the
GCE metadata host.  For maximum portability, don't rely on name
resolution.

We launched the default container found at https://index.docker.io/u/google/docker-registry/ on GCE and discovered the container could not resolve "metadata" and /etc/hosts is not writable in the image mentioned.  As a result, the public image was unusable for us.