interactive-sessions: implement configurable allowlist

[mdonadoni]

Let's add a configurable allowlist of docker images among which users can choose when starting a new interactive session.

Things to do:

• Decide how to let the admins customise this allowlist (most probably helm value?)
• Implement the actual allowlist checks in reana-workflow-controller
• Decide whether to let users choose the docker image also from the UI. In this case, the list needs to be exposed also in web interface.
• Bonus point: instead of a simple allowlist, consider making customisable also other details about the docker images (e.g. web port used, env variables needfed, etc.). This is a much bigger task, though.

Note that not all docker images are compatible with the way we currently deploy jupyter notebooks (e.g. they might expose a different port, etc.)