Adds [LinuxOnly] tag to conformance tests that cannot be run on Windows

Some of the tests cannot pass using Windows nodes due to various reasons:

- seLinuxOptions are not supported on Windows.
- Running as an UID / GID is not supported on Windows.
- file permissions work differently on Windows, and they cannot be set in
  the same manner as on Linux.
- individual files cannot be mounted in Windows Containers.
- Cannot create container using Linux image (e.g.: alpine) on Windows.

Because of this, it has been decided to use the "[LinuxOnly]" tag for the
tests which cannot run on Windows because of the mentioned reasons. This way,
when running tests using Windows nodes, those tests can simply be skipped by
adding the "[LinuxOnly]" tag to the ginkgo.skip argument.

test/e2e/common/kubelet.go

@@ -190,8 +190,9 @@ var _ = framework.KubeDescribe("Kubelet", func() {
 			Release : v1.13
 			Testname: Kubelet, pod with read only root file system
 			Description: Create a Pod with security context set with ReadOnlyRootFileSystem set to true. The Pod then tries to write to the /file on the root, write operation to the root filesystem MUST fail as expected.
+			This test is marked LinuxOnly since Windows does not support creating containers with read-only access.
 		*/
-		framework.ConformanceIt("should not write to root filesystem [NodeConformance]", func() {
+		framework.ConformanceIt("should not write to root filesystem [LinuxOnly] [NodeConformance]", func() {
 			isReadOnly := true
 			podClient.CreateSync(&v1.Pod{
 				ObjectMeta: metav1.ObjectMeta{