Investigate Steward CI

I recently found https://github.com/SAP/stewardci-core.

If I read it correctly, its basic promise is to get rid of Jenkins Master, while keeping Jenkins as the runtime. It uses Tekton as an executor for jenkinsfile-runner. This sounds interesting because - if it works well - this would allow us to keep the investment we made into the shared lib and all the Jenkinsfiles, while getting rid of that 5GB memory hog in every cd project.

My initial doubts are:

• Does jenkinsfile-runner support all features we need?
• How does shared library loading work (I think pre-baked works, but what if you want to pull a different branch?)
• How does that relate to agent images? I guess you wouldn't launch an agent pod, but ensure to already run on an image that can build your tech. How would one deal with multi-container pipelines?
• How does it work with plugins? Especially the openshift-sync plugin (e.g. credentials)? Do we need that at all anymore?
• How to deal with artefacts / attachments? They are stored with the Jenkins master right now.

So lots of open questions, but it'd be still interesting to see if this is a viable way forward (or an option for a smooth transition to something else) ...

FYI @metmajer @martsec @oalyman @gerardcl @clemensutschig

The issue names no files or tests. Review SAP/stewardci-core and compare its jenkinsfile-runner, shared-library, agent-image, plugin, and artifact behavior with the current OpenDevStack setup; done means documenting whether it is viable and answering the listed compatibility questions.