Fix race condition when joining nodes

[ereslibre]

What type of PR is this?
/kind bug

What this PR does / why we need it:
Despite we were checking for the kubelet kubeconfig file to be present, the
kubelet first writes this file and then the certificates the kubeconfig file
refers to. This represents a race condition in kubeadm in which when we confirm
that the kubelet's kubeconfig file is present we continue creating a clientset
out of it. However, the clientset creation will ensure that the certificates the
kubeconfig file refers to exist on the filesystem.

To fix this problem, not only wait for the kubelet's kubeconfig file to be
present, but also ensure that we can create a clientset ouf of it on our polling
process, while we wait for the kubelet to have performed the TLS bootstrap.

(cherry picked from commit a31c160463d8ed76b6a627e8ed879d72f79d9e08)

Backport of #72030

Which issue(s) this PR fixes:
Fixes kubernetes/kubeadm#1319

Does this PR introduce a user-facing change?:

NONE

[k8s-ci-robot]

@ereslibre: This PR is not for the master branch but does not have the cherry-pick-approved label. Adding the do-not-merge/cherry-pick-not-approved label.

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[k8s-ci-robot]

Hi @ereslibre. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ereslibre
To fully approve this pull request, please assign additional approvers.
We suggest the following additional approver: timothysc

If they are not already assigned, you can assign the PR to them by writing /assign @timothysc in a comment when ready.

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[ereslibre]

Closing due to not being the standard way of performing a cherry pick; just learnt about https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md