First pass at a great workflow RFC to rule them all.

I'm specifically ignoring the existence of Policyfiles and Berkshelf in the RFC, as both are implementation details. I think the right thing, long term, is to have Policyfile take the parts of Berkshelf we need, and have a mode that is back-compat to resolve Berksfiles. Then we build the workflows on top of Policyfile's primitives.