Adding Node Support to Test Kitchen/.kitchen.yml

This PR adds node support to TK and allows nodes to be specified in the .kitchen.yml; these changes are needed to support testing on multiple nodes (or something like it would be).  The current kitchen-metal requires these changes to function properly (see https://github.com/doubt72/kitchen-metal).

This is a fairly big change, so I'd expect peeps to have some comments/suggestions here before this is even considered for merging; points of contact for this are me (though my priority is another project at the moment) or @jkeisier (who is still working on Metal actively at this moment).