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.
Is that really safe?  The OpenStack CoreOS image is used for Eucalyptus too and I don't believe they support `/openstack` URL.  I personally use the OpenStack image on a cloud system that emulates the EC2 169.254.169.254, so if the OpenStack image switched to `/openstack` it would at least break me.