Make kubeadm able to consume CI images

[luxas]

Now that kubernetes/test-infra#1400 is implemented, we can go on and make it possible to do

$ kubeadm init --kubernetes-version master

or

$ kubeadm init --kubernetes-version HEAD

or

$ kubeadm init --kubernetes-version ci/latest

or

$ kubeadm init --kubernetes-version latest-ci

What do people prefer?
What would basically happen is that the repository prefix would automatically get adjusted to gcr.io/kubernetes-ci-images and the latest available tag would be fetched like this (pseudo-code):

$ curl -sSL dl.k8s.io/ci-cross/latest.txt
v1.8.0-alpha.1.608+992002689e3c54

Then kubeadm will go on and use those images super easily 🎉

This will unblock #238

[namliz]

That's awesome. I think it ought to be possible to override the repository prefix.

[luxas]

@Zilman It has been possible to override repository prefix since v1.5-ish, but it hasn't been well documented other than on the kubeadm reference docs on the site. It has been possible using the environment variable KUBE_REPO_PREFIX. However, that's being moved to the API in v1.8, ref: #326

However, being able to consume the latest CI builds automatically is exciting as well!

[ixdy]

cluster/get-kube.sh and kubetest use the ci/latest (or ci/latest-1.7, etc) form, so that seems most consistent with existing tooling.

[luxas]

@andrewrynhard volunteered to contribute this 🎉
Ask if you need anything

[kad]

consistency is a key. if we have "latest", "latest-1.7" and so on for releases, same should be for "ci/".

[kad]

@luxas @andrewrynhard if you don't mind, I'll take that issue, I have some early days drafts for it.

[andrewrynhard]

@kad Don't mind at all.

[luxas]

Great to have this issue closed, thanks @kad!