When rack awareness is configured, we add our own affinity rules:

• Pod anti-affinity to dsitribute the KAfka pods accross racks (zones)
• Node affinity to make sure the pods are scheduled only on nodes with the rack label

However, the node-affinity was added as a new NodeSelectorTerm. But with multiple NodeSelectorTerms, the terms are ORed. That means that this might mean that the user rules will be ignored. This PR fixes that and adds the rule as a new matchExpressions. Multiple matchExpressions are ANDed, so both our and users expression would be applied.