kubeadm: Use the v1.9 branch by default

[xiangpengzhao]

What this PR does / why we need it:
As per https://github.com/kubernetes/kubeadm/blob/master/docs/release-cycle.md, bump the default Kubernetes version to use in kubeadm right before v1.9.0-rc.1 (or maybe rc.0 ?) is cut.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
ref: #47440 #52085

Special notes for your reviewer:
/cc @luxas @kubernetes/sig-cluster-lifecycle-pr-reviews @kubernetes/kubernetes-release-managers

Release note:

NONE

[jamiehannaford]

/lgtm

[luxas]

/approve

@enisoc this PR must be merged right before you cut the release candidate

[luxas]

/approve no-issue

[k8s-github-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jamiehannaford, luxas, xiangpengzhao

Associated issue requirement bypassed by: luxas

The full list of commands accepted by this bot can be found here.

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

• cmd/kubeadm/OWNERS [luxas]

You can indicate your approval by writing /approve in a comment
You can cancel your approval by writing /approve cancel in a comment

[luxas]

/hold

This should be merged right before the release candidate is cut

[enisoc]

@luxas Can you elaborate on why this can't be merged now?

[luxas]

@enisoc Sure! Well, technically it can indeed, but the issue is that dl.k8s.io/release/stable-1.9.txt doesn't exist until v1.9.0 is cut, so we'd like to minimize the time kubeadm init at HEAD doesn't work out-of-the-box (as it tries to use a v1.9 version although there aren't any).

Makes sense to you?

[xiangpengzhao]

ping for Milestone update.

[xiangpengzhao]

/test pull-kubernetes-e2e-gce

[k8s-github-robot]

[MILESTONENOTIFIER] Milestone Pull Request Current

@jamiehannaford @timothysc @xiangpengzhao

Note: This pull request is marked as priority/critical-urgent, and must be updated every 1 day during code freeze.

Example update:

ACK.  In progress
ETA: DD/MM/YYYY
Risks: Complicated fix required

Pull Request Labels

• sig/cluster-lifecycle: Pull Request will be escalated to these SIGs if needed.
• priority/critical-urgent: Never automatically move pull request out of a release milestone; continually escalate to contributor and SIG through all available channels.
• kind/cleanup: Adding tests, refactoring, fixing old bugs.

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[luxas]

Let's merge this as we're getting close to releasing v1.9.0 :)

/hold cancel

[enisoc]

/retest

[xiangpengzhao]

/test pull-kubernetes-unit

[k8s-github-robot]

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

[k8s-github-robot]

Automatic merge from submit-queue (batch tested with PRs 56599, 56824, 56918, 56967, 56959). If you want to cherry-pick this change to another branch, please follow the instructions here.