kubeadm: Fix issue when kubeadm reset isn't working and the docker service is disabled

[luxas]

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 #43950

Special notes for your reviewer:

Release note:

kubeadm: Make `kubeadm reset` tolerant of a disabled docker service.

@mikedanese @jbeda @dmmcquay @pipejakob @yujuhong @freehan

[k8s-reviewable]

This change is 

[k8s-cherrypick-bot]

Removing label cherrypick-candidate because no release milestone was set. This is an invalid state and thus this PR is not being considered for cherry-pick to any release branch. Please add an appropriate release milestone and then re-add the label.

[0xmichalis]

Use var so you will continue to return nil from this method

[supereagle]

Yes, better to return nil if no data. Maybe it is enough to just remove this line.

[luxas]

ping @mikedanese @jbeda

[mikedanese]

I assume dockerCheck returns an error if the docker service is not running? Also this doesn't compile.

/lgtm

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: luxas, mikedanese

Needs approval from an approver in each of these OWNERS Files:

• cmd/kubeadm/OWNERS [luxas,mikedanese]

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[luxas]

@mikedanese An error is returned when the docker service is not active.
A warning is returned if the service is not active; which we should not care about when resetting things

[k8s-github-robot]

@k8s-bot test this [submit-queue is verifying that this PR is safe to merge]

[luxas]

@k8s-bot non-cri node e2e test this

[luxas]

@k8s-bot verify test this

@ixdy I think this build timed out

[ixdy]

yeah, looks like docker checked out or something.

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 43951, 43386)

[k8s-cherrypick-bot]

Commit found in the "release-1.6" branch appears to be this PR. Removing the "cherrypick-candidate" label. If this is an error find help to get your PR picked.