Automatic merge from submit-queue (batch tested with PRs 43951, 43386)
kubeadm: Fix issue when kubeadm reset isn't working and the docker service is disabled
**What this PR does / why we need it**:
If the docker service is disabled, the preflight check lib will return a warning.
That warning _should not_ matter when deciding whether to reset docker state or not.
The current code skips the docker reset if the docker service is disabled, which is a bug.
Also, `Check()` must not return a `nil` slice.
It should be added that I **really don't like what we have at the moment**, I'd love to discuss with the node team to add something to CRI that basically says, "remove everything on this node" so we can stop doing this. Basically, kubeadm could talk to the specified socket (by default dockershim.sock), and call the CRI interface and say that everything should be cleaned up. This would then be cross-CRI-implementation at the same time and would work if you're using rkt, cri-o or whatever.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*:
helps in https://github.com/kubernetes/kubernetes/issues/43950
**Special notes for your reviewer**:
**Release note**:
```release-note
kubeadm: Make `kubeadm reset` tolerant of a disabled docker service.
```
@mikedanese @jbeda @dmmcquay @pipejakob @yujuhong @freehan
e6dc13408f