Use worker nodes for WaitForStableCluster()

WaitForStableCluster() checks all pods run on worker nodes, and the
function used to refer master nodes to skip checking controller plane
pods.
GetMasterAndWorkerNodes() was used for getting master nodes, but the
implementation is not good because it usesDeprecatedMightBeMasterNode().

This makes WaitForStableCluster() refer worker nodes directly to avoid
using GetMasterAndWorkerNodes().

test/e2e/scheduling/preemption.go

@@ -97,11 +97,14 @@ var _ = SIGDescribe("SchedulerPreemption [Serial]", func() {
 		}
 
 		e2enode.WaitForTotalHealthy(cs, time.Minute)
-		masterNodes, nodeList, err = e2enode.GetMasterAndWorkerNodes(cs)
+		nodeList, err = e2enode.GetReadySchedulableNodes(cs)
 		if err != nil {
 			framework.Logf("Unexpected error occurred: %v", err)
 		}
 		framework.ExpectNoErrorWithOffset(0, err)
+		for _, n := range nodeList.Items {
+			workerNodes.Insert(n.Name)
+		}
 
 		err = framework.CheckTestingNSDeletedExcept(cs, ns)
 		framework.ExpectNoError(err)