spike: Support dynamic product image selection

[Techassi]

Caution

This is a spike and this will most likely not be merged, but the individual changes will come in through operator-templating.

Part of stackabletech/issues#716.

This is a spike PR to test out a bunch of things:

• Publish to multiple registries (This seems a little flaky, also new stuff is private by default)
• Merge registry specific Helm values (CI run)
• Propagate the registry and repository to the operator (tested with make run-dev and make helm-install)

[Techassi]

Most (if not all) of these changes will go into operator-templating.

[Techassi]

CI will break, because I pointed at a local stackable-operator version with the required changes.

[Techassi]

Closing, final implemenation was done in #782 and #784.