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.
We're not using YARD and don't have much docs at all right now. I'm generally just doing a `git grep TODO` regularly to find and fix these in the short-term. I'm planning to address this particular TODO when we add UI integration with the various policyfile bits.