kubeadm: Remove .ImagePullPolicy from the v1alpha2 API

[luxas]

What this PR does / why we need it:
So with kubeadm config images list/pull I don't think we need this anymore. Also I don't like this being in our API, as I think the purpose of why it's there can be achieved in other ways.
Instead, I propose to set this explicitely to IfNotPresent, and tell the user to prepull the images with kubeadm config images pull in case of an airgapped env (or docker load ofc) or he/she wants to achieve what imagePullPolicy: Always would do. If the images are already cached locally, IfNotPresent translates to the same as Never, i.e. don't pull (for ppl with no internet connection).

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Part of cleaning up the API kubernetes/community#2131

Special notes for your reviewer:
This basically reverts: #58960

Release note:

[action required] kubeadm: The `.ImagePullPolicy` field has been removed in the v1alpha2 API version. Instead it's set statically to `IfNotPresent` for all required images. If you want to always pull the latest images before cluster init (like what `Always` would do), run `kubeadm config images pull` before each `kubeadm init`. If you don't want the kubelet to pull any images at `kubeadm init` time, as you for instance don't have an internet connection, you can also run `kubeadm config images pull` before `kubeadm init` or side-load the images some other way (e.g. `docker load -i image.tar`). Having the images locally cached will result in no pull at runtime, which makes it possible to run without any internet connection.

@kubernetes/sig-cluster-lifecycle-pr-reviews @rosti @liztio @chuckha

[rosti]

I don't rely on this config option anymore, so it's OK by me.

[rosti]

typo? "mitigate"? :)

[rosti]

kubelet defaults to IfNotPresent if this is not set in the static pod YAML. Thus setting it here explicitly can be omitted (although it doesn't do any harm to be set explicitly here).

[chuckha]

I think this makes sense and cleans up a field in the API 👍.

One concern I have with this path (which admittedly is beyond this PR) is that the scope of preflight checks will expand when we start pulling images with checks. To me, that feels like they are no longer checks but preflight actions. It might be that checks are already allowed to be mutative actions, but my assertion based on the name is that they were seeing if the system was in the right state and reporting errors, not fixing it for the user.

[chuckha]

typo in 'miligate', I assume you mean 'mitigate'. Also drop the word 'in'

[timothysc]

ImagePullPolicy is useful, some folks have strict requirements to pull from their own registry (always).

I'm ok with this feature addition.

/hold

[timothysc]

k, we just need to document the change as part of the workflow.

/lgtm
/approve

[k8s-ci-robot]

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[luxas]

/retest

[luxas]

ImagePullPolicy is useful, some folks have strict requirements to pull from their own registry (always).

We discussed this with @timothysc, and kubeadm config images pull && kubeadm init equals Always, so with that flow (which we will document), we don't need this and can remove to get a cleaner struct.

Also noteworthy is that kubernetes/kubeadm#825 is coming to prepull images in the preflight checks if images don't exist locally already. This will help us get from the place where we wait for a long time for the kubelet to pull the images, with poor UX as kubeadm can hang if there's no internet connection. These are slightly related, so hence the mentioned here.

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 59414, 64096). If you want to cherry-pick this change to another branch, please follow the instructions here.