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.
sure, it's just a bit trickier and more work b/c cookbook-omnifetch is a different gem. I seem to remember active model providing an abstract test case for API compliance, but I don't remember how they did it, will have to look that up. On the other hand, there's only one implementation in the wild (this one) and if I integrate berks with cookbook-omnifetch, there would be two. So I'm not sure if it's worth the effort.