Adding an e2e test on GMSA support

The previously existing e2e GMSA test really only tests a small part of the
whole GMSA set up process, namely that once the API has inlined the GMSA
contents in the pod's spec, and sent that to a worker's kubelet, then the
kubelet passes that down to the runtime.

This new test, in contrast, really tests the whole thing, i.e. deploying the
admission webhook, then deploying a GMSA custom resource, and using that
resource within a pod.

The downside of this test though, is that it does need to make a lot of
assumptions about the cluster it runs against, notably that it runs on a worker
node that's already been joined to a working Active Directory domain (there are
other assumptions, all documented at the beginning of the test file); for that
reason, it is only intended to ever be run against an AKS cluster with the
custom AKS extension from
https://github.com/kubernetes-sigs/windows-testing/pull/98.

Note that this test doesn't aim at testing every edge-case, such as
a pod trying to use a GMSA it doesn't have access to; the webhook has
its own tests for these. This test's goal is to ensure the happy path
doesn't break.

Signed-off-by: Jean Rouge <rougej+github@gmail.com>

test/e2e/windows/utils.go

@@ -0,0 +1,44 @@
+/*
+Copyright 2019 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package windows
+
+import (
+	"io"
+	"io/ioutil"
+	"net/http"
+
+	"github.com/pkg/errors"
+)
+
+// downloadFile saves a remote URL to a local temp file, and returns its path.
+// It's the caller's responsibility to clean up the temp file when done.
+func downloadFile(url string) (string, error) {
+	response, err := http.Get(url)
+	if err != nil {
+		return "", errors.Wrapf(err, "unable to download from %q", url)
+	}
+	defer response.Body.Close()
+
+	tempFile, err := ioutil.TempFile("", "")
+	if err != nil {
+		return "", errors.Wrapf(err, "unable to create temp file")
+	}
+	defer tempFile.Close()
+
+	_, err = io.Copy(tempFile, response.Body)
+	return tempFile.Name(), err
+}