Fallback to plain metadata file for community cookbooks

In case an Opscode cookbook site archive doesn't contain a metadata.json,
this change will add a fallback to the plain metadata.rb. It's necessary
for alternative cookbook site implementations.

I started to write a community site compatible cookbook location, which I hope to finish soon. The provided tarballs don't include a `metadata.json` though (as they just provide the plain cookbook source), so I need this change. While this change might not ever be necessary for the Opscode cache_builder, I don't think it makes sense to create a new worker type as the rest of the interface is identical to Opscode.

@reset It seems you removed the support for `metadata.rb` in 28788d9, was there any particular reason for that change?