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.
Yea a shared_example is [what I used in omnibus](https://github.com/opscode/omnibus-ruby/blob/ab7a856bc8dfc0022990c4d878840b2d5702647c/spec/unit/project_spec.rb#L10-L34). Don't get me wrong, I think we should keep the docs. But if we are declaring this is "public", it should be part of our automated test suite(s).