Remove .chef-zero submodule
Clean up unused config files in favor of Test Kitchen
Fix FC019: Access node attributes in a consistent manner

We use strings in other attributes in the cookbook, thus should use strings
here.
Clean up Gemfile

* chef & chef-zero come along with Test-Kitchen
* rspec-mocks & rspec-core come along with chefspec
* Add sane version constraints to avoid breaking changes in the future
Add Rakefile for easier testing etc.