Required properties on models in k8s spec are ignored

[obmarg]

For API definitions we use the required property in the k8s spec to decide where the argument should go.

Models also have required properties, but we don't do anything with them. We should probably do something with them. At the very least update our documentation to show which properties are required. Perhaps even enforce them with @enforce_keys, though this could make building models more awkward so may not be worth it...