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.
From what I understand, most tools hard code http://169.254.169.254/2009-04-04/meta-data/ and I believe that works on OpenStack, but I don't have an env to test on.