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.
actually is this the right URL? Don't you need `/openstack/latest/meta-data.json`?

My reading of [this page](http://docs.openstack.org/grizzly/openstack-compute/admin/content/metadata-service.html) is that the JSON endpoint is under `/openstack/...`