refactor(kitchen): prefer `kitchen.yml` to `.kitchen.yml`

* Quoting from https://kitchen.ci/docs/getting-started/kitchen-yml/:
  - As of test-kitchen 1.21.0, we now prefer `kitchen.yml` over `.kitchen.yml`.
  - This preference applies to `kitchen.local.yml` as well.
  - This is backward compatible so the dot versions continue to work.