provisioners/chef: Add `chef_zero` provisioner

Add Chef Zero provisioner using `local_mode` configuration for
chef-solo/chef-client.
website/docs: Add documentation for the Chef Zero provisioner
Use require_relative for faster loading

require_relative is an order of magnitude faster on Windows
Make Chef Zero lowercase in validation (conforms to other keys)
Add tests for Chef base Config
Remove :focus tag (accidentally left from previous commit)
Capitalize Chef Server in errors
Check for empty chef_server_url and validation_key_path
Add tests for Chef Client config
Properly handle empty/nil values in Chef Solo config
Add tests for Chef Solo config
Add tests for Chef Zero config
Use spaces in website docs
Tabs -> Spaces
Merge branch 'sethvargo/chef_and_tests'