Automatic merge from submit-queue (batch tested with PRs 37328, 38102, 37261, 31321, 38146)

Fixes flake: wait for dns pods terminating after test completed

From #37194. Based on #36600. Please only look at the second commit.

As mentioned in [comment](https://github.com/kubernetes/kubernetes/issues/37194#issuecomment-262007174), "DNS horizontal autoscaling" test does not wait for the additional pods to be terminated and this may lead to the failure of later tests.

This fix adds a wait loop at the end of the serial test to ensure the cluster recovers to the original state. In the non-serial test it does not wait for the additional pods terminating because it will not affect other tests, given they are able to be run simultaneously. Plus wait for pods terminating will take certain amount of time.

Note this only fixes certain case of #37194. I noticed there are other failures irrelevant to dns autoscaler. LIke [this one](https://k8s-gubernator.appspot.com/build/kubernetes-jenkins/logs/ci-kubernetes-e2e-gci-gce-serial/34/).

@bprashanth @Random-Liu