Priority node selector should be compatible with the pod template

[aermakov-zalando]

Expected Behavior

Creating a cluster definition where the pod template doesn't include either a nodeSelector or an affinity that's compatible with the priority node selector should either result in an error, or an automatically modified pod template so the cluster behaves correctly during the updates.

Actual Behavior

If the users don't define a nodeSelector or an affinity in the pod spec, but define a priority node selector in the operator configuration, it's highly likely that a rolling cluster update will not be handled correctly. For example, if the cluster management software is currently draining node A and the cluster pods live on nodes A, B and C that are scheduled to be drained, it's possible that the operator will just keep deleting pod on node B that would be rescheduled to the same node again and again, and will not actually proceed to node A.

Steps to Reproduce the Problem

1. Setup an operator with a priority node selector.
2. Setup a cluster whose pod template doesn't include the priority node selector at all.
3. Observe broken behaviour during cluster updates.