Add Chef Zero provision + improve Chef test coverage

/cc @tmatilai 

Closes #3853 
I usually just do a `context "defaults" ` section for all of these to put them all in one place. See here: https://github.com/mitchellh/vagrant/blob/5483cadb87e6a6a572ed36340a09377b6baa626e/test/unit/plugins/providers/docker/config_test.rb#L38