/kind feature

Describe the solution you'd like
The multi-stage cluster addons (SovereignCloudStack/cluster-stack-operator#64) need support of the csctl to properly generate release assets for testing and also later for production.

We want to support both the old way without multi-stage addons, and the new way. We can find that out based on whether there is a clusteraddon.yaml file (which indicates the new way).