Merge pull request #62951 from dims/support-nsenter-better-in-non-systemd-envs

Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Support nsenter in non-systemd environments

**What this PR does / why we need it**:

In our CI, we run kubekins image for most of the jobs. This is a
debian image with upstart and does not enable systemd. So we should

* Bailout if any binary is missing other than systemd-run.
* SupportsSystemd should check the binary path to correctly
  identify if the systemd-run is present or not
* Pass the errors back to the callers so kubelet is forced to
  fail early when there is a problem. We currently assume
  that all binaries are in the root directory by default which
  is wrong.


**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```

cmd/kubelet/app/server.go

@@ -335,7 +335,10 @@ func UnsecuredDependencies(s *options.KubeletServer) (*kubelet.Dependencies, err
 	var writer kubeio.Writer = &kubeio.StdWriter{}
 	if s.Containerized {
 		glog.V(2).Info("Running kubelet in containerized mode")
-		mounter = mount.NewNsenterMounter()
+		mounter, err = mount.NewNsenterMounter()
+		if err != nil {
+			return nil, err
+		}
 		writer = &kubeio.NsenterWriter{}
 	}