kubeadm: prepull Etcd image only if required

[bart0sh]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Prepull Etcd image only if external Etcd is not used and
--etcd-upgrade=false is not specified.

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

Fixes: kubernetes/kubeadm#1214

Does this PR introduce a user-facing change?:

kubeadm pre-pulls Etcd image only if external Etcd is not used and
--etcd-upgrade=false is not specified

[bart0sh]

/cc youyongsong

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[bart0sh]

/cc @youyongsong

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[rosti]

Actually, the problem here is a bit more fundamental I think. During init/join we have a pre-flight check that pulls images sequentially, using the GetAllImages function to get the image list. This results in control plane images, kube-proxy, etcd (if local), DNS and pause images to be pre-pulled.

With upgrade, things are different. Only control plane and etcd (if local) images are pre-pulled via the PrepullImagesInParallel func even though DNS and kube-proxy are upgraded too. Pause is not dealt with during upgrade (even though its version can change too).

I do think, that we can have some code sharing between init/join and upgrade. Probably I should spend some time preparing a cleanup PR for this.

[neolit123]

/priority important-soon

[neolit123]

/lgtm
/retest

@rosti @bart0sh
follow up PRs are welcome!

[bart0sh]

/assign @timothysc

[timothysc]

/lgtm
/approve
/milestone v1.13

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[APPROVALNOTIFIER] This PR is APPROVED

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