Core Group Provider and Dscl provider (Mac OS) changes for CHEF-3297

This PR includes the required Core Group Provider and Dscl provider changes for CHEF-3297. For more information see this:

https://gist.github.com/sersut/ff4574d61b9db883691f 

@lamont-granquist / @danielsdeleo thoughts? 

PS: Note that this PR targets CHEF-3297 branch in which I would like to combine all required changes which will make it easier to port these to 10stable.