e2e: grab controller and scheduler metrics via port forwarding

The previous approach with grabbing via a nginx proxy had some
drawbacks:
- it did not work when the pods only listened on localhost (as
  configured by kubeadm) and the proxy got deployed on a different
  node
- starting the proxy raced with starting the pods, causing
  sporadic test failures because the proxy was not set up
  properly unless it saw all pods when starting the e2e.test
- the proxy was always started, whether it is needed or not
- the proxy was left running after a test and then the next
  test run triggered potentially confusing messages when
  it failed to create objects for the proxy

The new approach is similar to "kubectl port-forward" + "kubectl get
--raw". It uses the port forwarding feature to establish a TCP
connection via a custom dialer, then lets client-go handle TLS and
credentials.

Somehow verifying the server certificate did not work. As this
shouldn't be a big concern for E2E testing, certificate checking gets
disabled on the client side instead of investigating this further.

test/e2e/storage/testsuites/volumemode.go

@@ -115,7 +115,7 @@ func (t *volumeModeTestSuite) DefineTests(driver storageframework.TestDriver, pa
 
 		// Now do the more expensive test initialization.
 		l.config, l.driverCleanup = driver.PrepareTest(f)
-		l.migrationCheck = newMigrationOpCheck(f.ClientSet, dInfo.InTreePluginName)
+		l.migrationCheck = newMigrationOpCheck(f.ClientSet, f.ClientConfig(), dInfo.InTreePluginName)
 	}
 
 	// manualInit initializes l.VolumeResource without creating the PV & PVC objects.