kubelet: only register api source when connecting

[roboll]

What this PR does / why we need it:
before this change, an api source was always registered, even when there
was no kubeclient. this lead to some operations blocking waiting for
podConfig.SeenAllSources to pass, which it never would.

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 #59275

Special notes for your reviewer:

Release note:

Fix the bug where kubelet in the standalone mode would wait for the update from the apiserver source.

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[roboll]

/assign @tallclair

[timothysc]

/sig node

[timothysc]

It generally /lgtm given your description of the problem, but the original code mod was part of a recommendation from @kubernetes/sig-node-pr-reviews

[yujuhong]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: roboll, yujuhong

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[yujuhong]

@roboll could you also create a cherrypick PR to patch 1.9? Thanks!

[k8s-github-robot]

[MILESTONENOTIFIER] Milestone Pull Request Labels Incomplete

@roboll @timothysc @yujuhong

Action required: This pull request requires label changes. If the required changes are not made within 2 days, the pull request will be moved out of the v1.9 milestone.

priority: Must specify exactly one of priority/critical-urgent, priority/important-longterm or priority/important-soon.

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[yujuhong]

It generally /lgtm given your description of the problem, but the original code mod was part of a recommendation from @kubernetes/sig-node-pr-reviews

I think it's just a bug and/or oversight. Not sure whether this was recommended explicitly....

[k8s-github-robot]

/test all [submit-queue is verifying that this PR is safe to merge]

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 59276, 51042, 58973, 59377, 59472). If you want to cherry-pick this change to another branch, please follow the instructions here.

[k8s-cherrypick-bot]

Commit found in the "release-1.9" branch appears to be this PR. Removing the "cherrypick-candidate" label. If this is an error find help to get your PR picked.