I'm not quite sure why this is the focus of commenting. [This code already exists in master util.rb](https://github.com/test-kitchen/test-kitchen/blob/533dcb5ef16fef80431cc592db59ee2a3af51e61/lib/kitchen/util.rb#L109-L219)

The only thing this PR does is add a layer of indirection for which bash script to run for installing chef or anything a user needs. It sounds like there is some controversy around using this bash script. My goal was to maintain existing code while introducing these changes.

I ask that we table the discussion about shell_helpers for another PR, since they aren't really an addition. Does that sound reasonable?