Support MixedMode for acceptance tests

[leochr]

Add a new option named 'MixedMode' for the operator install mode, so the Operator can be installed using a different install mode on each of the architecture. For example, OwnNamespace on X, AllNamespaces on P and SingleNamespace on Z. This way all install modes are covered in one build (without having to install all install modes on each architecture).

If it can be implemented, we should randomly assign the install mode for each architecture - just to detect any potential platform + install mode specifc issues.