[Federation] Make --dns-provider mandatory for kubefed

[marun]

Targets #40757

I thought about adding a test for this but I decided it wasn't worth it. There's too much setup involved in being able to run Complete for such a simple change.

Release note:

The --dns-provider argument of 'kubefed init' is now mandatory and does not default to `google-clouddns`. To initialize a Federation control plane with Google Cloud DNS, use the following invocation: 'kubefed init --dns-provider=google-clouddns'

cc: @kubernetes/sig-federation-pr-reviews @madhusudancs

[k8s-reviewable]

This change is 

[madhusudancs]

@marun this deserves a release note. Please add it.

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Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[madhusudancs]

/lgtm
/approve

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Review status: all files reviewed at latest revision, all discussions resolved, some commit checks pending.

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Comments from Reviewable

[marun]

@madhusudancs Is the PR title acceptable as a release note? Or can you point me to docs for what is expected?

[madhusudancs]

@marun I usually expect people to write a separate release note unless it is really unnecessary.

PR title, like commit logs/messages, need to be imperative, i.e. an instructional message describing the "why of your change". This post - https://chris.beams.io/posts/git-commit/ is a nice write about writing good commit logs, which also applies to PR titles.

On the other hand, a release note should be informative. It is an information to the user who does not really care about the details or "why of the change".

There are cases where these two are the same, but not in this PR. Your PR title is good as a, well, PR title. But for a user reading our release notes, it is not that useful. So I think this still deserves a release note. I would write the release note for this PR as:

`kubefed init`'s --dns-provider is now a mandatory flag and does not default to `google-clouddns` anymore. If you were previously using Google Cloud DNS and depending on the defaulting behavior, you should now explicitly pass `--dns-provider=google-clouddns` in the future invocations of `kubefed init`.

Btw, this is a release-note-action-required.

[madhusudancs]

@k8s-bot gci gce e2e test this
@k8s-bot non-cri e2e test this

[madhusudancs]

/lgtm

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

The following people have approved this PR: k8s-merge-robot, madhusudancs, marun

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

• federation/OWNERS [madhusudancs]

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[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 41937, 41151, 42092, 40269, 42135)