Install cookbooks from Policyfile.lock.json

Adds library code that can install cookbooks directly from the lockfile. There is no end-user interface for this behavior (that will come once some other prerequisite functionality has been implemented), but the scripts in the test-scripts directory exercise the code in a real-world-ish scenario.
Maybe I'm not following correctly, but it doesn't look like the DSL is explicitly evaluting the file in the right context (again, I could be wrong). I really like the approach of making methods explicitly public like we did in Omnibus. It's one less level of abstraction