Add validation for modifications to availability zones

Updating Availability Zone is not allowed for Azure AKS.

Issue: #669
(cherry picked from commit cb67801)

controller/aks-cluster-config-handler.go

@@ -944,6 +944,9 @@ func (h *Handler) updateUpstreamClusterState(ctx context.Context, config *aksv1.
 					logrus.Debugf("config: %s; upstream: %s", np.Mode, upstreamNodePool.Mode)
 					updateNodePool = true
 				}
+				if np.AvailabilityZones != nil && !reflect.DeepEqual(np.AvailabilityZones, upstreamNodePool.AvailabilityZones) {
+					logrus.Errorf("Changing availability zones for node pool [%s] in cluster [%s (id: %s)] is not permitted", aks.String(np.Name), config.Spec.ClusterName, config.Name)
+				}
 			} else {
 				logrus.Infof("Adding node pool [%s] for cluster [%s (id: %s)]", aks.String(np.Name), config.Spec.ClusterName, config.Name)
 				updateNodePool = true