[kubeadm] Print required flags when running kubeadm upgrade plan

[xlgao-zju]

What this PR does / why we need it:
print required flags when running kubeadm upgrade plan

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Close kubernetes/kubeadm#935

Special notes for your reviewer:
/assign @chuckha
/assign @neolit123

Release note:

kubeadm: print required flags when running kubeadm upgrade plan

[neolit123]

thank you @xlgao-zju
/lgtm

[neolit123]

/kind bug
@kubernetes/sig-cluster-lifecycle-pr-reviews

potential cherry-pick?

[neolit123]

@chuckha @xlgao-zju
i think we should handle --allow-release-candidate-upgrades as well?

[xlgao-zju]

i think we should handle --allow-release-candidate-upgrades as well?

@neolit123 I think we can handle RC by using --allow-experimental-upgrades too?

--allow-experimental-upgrades        Show unstable versions of Kubernetes as an upgrade alternative and allow upgrading to an alpha/beta/release candidate versions of Kubernetes.
--allow-release-candidate-upgrades   Show release candidate versions of Kubernetes as an upgrade alternative and allow upgrading to a release candidate versions of Kubernetes.

[neolit123]

that's true.

if the user types:

kubeadm upgrade plan --allow-release-candidate-upgrades

to my understanding it would print:

You can now apply the upgrade by executing the following command:

        kubeadm upgrade apply <some-RC-version> --allow-experimental-upgrades

so it's a question if we want to print the exact --allow-..... flag that the user initially entered.
--allow-experimental-upgrades would be valid.

[xlgao-zju]

Maybe this is better instead of using the user's initial flag?

When the version is RC:

You can now apply the upgrade by executing the following command:

        kubeadm upgrade apply <some-RC-version> --allow-release-candidate-upgrades

when the version is some other unstable version:

You can now apply the upgrade by executing the following command:

        kubeadm upgrade apply <some-other-unstable-version> --allow-experimental-upgrades

[xlgao-zju]

@neolit123 And using the user's initial flag will make it difficult to deal with the test cases....

[chuckha]

@xlgao-zju we should support both flags, I like the idea of printing the correct one based on the suggested version instead of printing the flag the user passed in even though that's the only time we'll see this output.

[xlgao-zju]

@chuckha @neolit123 Updated based on:

the correct one based on the suggested version

PTAL

[neolit123]

@xlgao-zju
thanks, the change seems good.

waiting on @chuckha for confirmation.

[luxas]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: luxas, neolit123, xlgao-zju

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[k8s-github-robot]

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

[k8s-github-robot]

Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions here.