kubeadm: fix broken kubeadm init --config flag

[mattmoyer]

What this PR does / why we need it:
This code was changed in ea19649 (#43558) to validate that --config wasn't passed along with other flags. Unfortunately, the implementation was checking PersistentFlags(), which was not parsed at the point it was being validated.

The fix is to use Flags() instead, which contains the expected data.

Which issue this PR fixes fixes kubernetes/kubeadm#345

Special notes for your reviewer:
This is a regression that was cherry picked (#48577) into the release-1.7 branch. We should fix this before the next 1.7 point release.

This is awkward to unit test without restructuring the code. I think this command parsing code would be a good candidate for some higher level tests.

Release note:

Fix a regression that broke the `--config` flag for `kubeadm init`.

/assign @luxas

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[mattmoyer]

We need to cherry pick this onto release-1.7 to avoid a regression between 1.7.0 and 1.7.1 (unreleased).

[luxas]

/ok-to-test
/lgtm
@wojtek-t unfortunate, but we have to cherrypick this to v1.7.1. PTAL

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

Associated issue: 345

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[wojtek-t]

@luxas - 1.7.1 is already released. It's too late to modify 1.7.1 ...

[wojtek-t]

There was some accelerated 1.7.1 release that I wasn't really involved into.
That said, there will be 1.7.2 that I will cut next week and I will keep this PR in mind.

[luxas]

There was some accelerated 1.7.1 release that I wasn't really involved into.

Oh, hadn't noticed that. Well, ok.

That said, there will be 1.7.2 that I will cut next week and I will keep this PR in mind.

Thanks!

[luxas]

/retest

[k8s-github-robot]

Automatic merge from submit-queue

[k8s-cherrypick-bot]

Commit found in the "release-1.7" branch appears to be this PR. Removing the "cherrypick-candidate" label. If this is an error find help to get your PR picked.