fix #542

This will try to retrieve the 'latest' tag. If that fails,
we will attempt to create a 'v0' layer that acts as an empty
placeholder for metadata. The initial config will still result
as `v2` on the registry.
Merge pull request #625 from opdemand/542-config-vars

Looks good to me. (Didn't we used to get coveralls reports on PRs? Whatevs.)