kubernetes: support image "kind" and deprecate the debug boolean

This patch does two, closely related things:

First, it adds the mw_kind field to DeploymentsConfig items.

At a high level, "kind" is analogous to an image build target - i.e., a procedure / recipe that is used by the make-container-image scripts to produce an image. Think, e.g., the debug image.

In contrast, "flavour" continues to represent the particular combination of parameters used to instantiate the image - e.g., build args, like the PHP version).

To minimize coupling between scap and make-container-image, mw_kind is treated as an opaque key into the build report (e.g., "debug-image"), very similar to how flavour is already treated.

To compensate for the fact that we're introducing "another opaque key to get wrong", we now collect all helmfile release values updates before we attempt to apply any of them, so a misconfiguration is surfaced early.

This happens after image build in order to avoid surprises in the case where mediawiki-deployments.yaml is updated at the time of the first scap run after a new kind is added.

Second, it begins deprecation of the debug boolean, in favor of explicitly selecting the debug-image kind and testservers stage.

Once mediawiki-deployments.yaml and train-dev's equivalant are updated, I'll remove support for debug.

Bug: T389499