add options to force enable/disable cluster-maxunavailable

This MR introduces CI options to allow testing the cluster-maxunavailable behavior.

(used in sylva-projects/sylva-core!6523)

This can be merged anytime and won't break anything, it will only have effect into CI pipeline that use "🛠️ cluster-maxunavailable-force-disable" or "🛠️ cluster-maxunavailable-force-enable".