Surround RSpec tests with "describe" blocks as in test-kitchen's spec

Examples:

 - [chef_base_spec](https://github.com/test-kitchen/test-kitchen/blob/master/spec/kitchen/provisioner/chef_base_spec.rb)

 - [chef_solo_spec](https://github.com/test-kitchen/test-kitchen/blob/master/spec/kitchen/provisioner/chef_solo_spec.rb)

 - [chef_zero_spec](https://github.com/test-kitchen/test-kitchen/blob/master/spec/kitchen/provisioner/chef_zero_spec.rb)