WIP: [provisioner/chef-zero] Add support for chef-zero provisioner.

This is a work in progress. First attempt was #2570. 

I took a second crack at adding a Chef Zero provisioner. This formally adds the `chef_zero` provisioner which merely adds the option for a binary attribute on the `chef-client` configuration. For now it looks like we do not want to deprecate chef-solo and its much cleaner to know you're using a separate provisioner for now.