Remove outdated method comment
Actually use custom cookbook in test.

Test for custom `generator-cookbook` wasn't forcing options to be
parsed, so it did not test the behavior it was supposed to test.
Allow generator cookbooks with arbitrary names

* generator cookbooks can be named anything
* the argument to the `--generator-cookbook` option is now the path to
  the cookbook, rather than its parent directory (old way works for
  compatibility). This should be more obvious to new users.
Wrap exceptions from ChefRunner in ChefRunnerError
Extract custom generator tests to shared.
Merge branch 'custom-generator-names'