metadata-service: Add new datasource to download metadata

Add a new metadata-service data-source which is capable of fetching meta-data.json or walking the metadata directory if the former doesn't exist. Move the old metadata-service to a new url data-source to be backwards-compatible.
This is fine. When we invoke coreos-cloudinit on openstack, we'll use `--from-metadata-service=http://169.254.169.254/openstack`.