Allow Berkshelf to handle cookbook loading

In #56 I documented an issue I had that turned out to be another chef environment leaking into the elasticsearch tests.  It was from specifying '..' as a cookbook path in the Vagrantfile.

It seems to me that we should just allow Berkshelf to handle all of the cookbook loading and remove the `chef.cookbooks_path` setting altogether.

With this change, all tests pass on master in my environment.