Add minimal metadata for modern publishers

The publishers from modern Omnibus expect a minimal set of metadata in
order to properly publish artifacts. This commit adds the missing
metadata values.

/cc @opscode/release-engineers 

This build has successfully passed CI testing also:
http://wilson.ci.opscode.us/job/chef-server-trigger-ad_hoc/11/downstreambuildview/
A trailing comma here would be most useful for future diffs of america