Enable the ability to pass a inspec config.yml file to kitchen

[aaronlippold]

As an end user, I would like to be able to pass the --config <myconfig> to InSpec on a validation run.

I think we can just steal from the --input_files code, and I will review to see if I can push a quick PR but it may be just as fast for one of the maintainers to just push it as you know the codebase better than I.