Enforce consistent YAML style/structure via a YAML linter

* Use automation to enforce consistent code style and structure across our YAML files (i.e., run a linter on pull-requests).
* Why is this important?
* See the number of style (e.g., spacing/indent) related diffs in this pull-request: https://github.com/GoogleCloudPlatform/kubernetes-engine-samples/pull/1685/files. Enforcing a single, consistent style for YAML files would allow developers (and reviewers) to focus more on just the changes that primarily impact the functionality of the YAML files.
* This would also improve the overall "readability" of our YAML samples.
* Additional thoughts:
* We should default to common community practices/standards (so that our samples are more copy-pastable).