Automatically download missing kube binaries in kube-up/kube-down.

[ixdy]

What this PR does / why we need it: some users extract kubernetes.tar.gz and then immediately call cluster/kube-up.sh without first calling the new cluster/get-kube-binaries.sh script. As a result, the cluster fails to start, but it's not immediately clear why binaries are missing.

This PR streamlines this workflow by detecting this condition and prompting the user to download necessary binaries (using cluster/get-kube-binaries.sh).

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #38725

Release note:

Since `kubernetes.tar.gz` no longer includes client or server binaries, `cluster/kube-{up,down,push}.sh` now automatically download released binaries if they are missing.

cc @arun-gupta @christian-posta

[k8s-reviewable]

This change is 

[ixdy]

$ tar xzf kubernetes.tar.gz
$ cd kubernetes
$ KUBERNETES_RELEASE_URL=file:///tmp/release-tars ./cluster/kube-up.sh 
... Starting cluster in us-central1-b using provider gce
... calling verify-prereqs
... calling verify-kube-binaries
!!! kubectl appears to be broken or missing
!!! Cannot find kubernetes-server-linux-amd64.tar.gz
Required binaries appear to be missing. Do you wish to download them? [Y/n]

Kubernetes release: v1.6.0-alpha.0.2098+cbee65a6d2b969
Server: linux/amd64  (to override, set KUBERNETES_SERVER_ARCH)
Client: linux/amd64  (autodetected)

Will download kubernetes-server-linux-amd64.tar.gz from file:///tmp/release-tars/v1.6.0-alpha.0.2098+cbee65a6d2b969
Will download and extract kubernetes-client-linux-amd64.tar.gz from file:///tmp/release-tars/v1.6.0-alpha.0.2098+cbee65a6d2b969
Is this ok? [Y]/n

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  264M  100  264M    0     0   940M      0 --:--:-- --:--:-- --:--:--  940M

md5sum(kubernetes-server-linux-amd64.tar.gz)=e64efb5461d4e62a87e349fcaadf7e56
sha1sum(kubernetes-server-linux-amd64.tar.gz)=e742eac127b46aba4ce4434a0b5af91a95f1884e

  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 20.9M  100 20.9M    0     0   991M      0 --:--:-- --:--:-- --:--:--  991M

md5sum(kubernetes-client-linux-amd64.tar.gz)=ddf2a4317e23dcbff5d6d38caca757fa
sha1sum(kubernetes-client-linux-amd64.tar.gz)=95fb61aa11d05a5322c2c159bf652333b64cc8c1

Extracting /tmp/k/kubernetes/client/kubernetes-client-linux-amd64.tar.gz into /tmp/k/kubernetes/platforms/linux/amd64
Add '/tmp/k/kubernetes/client/bin' to your PATH to use newly-installed binaries.
... calling kube-up
Your active configuration is: [default]
...

[ixdy]

If you want to test this fix yourself (linux/amd64 only):

$ export KUBERNETES_RELEASE_URL=https://storage.googleapis.com/kubernetes-release-pull/ci/pull-kubernetes-e2e-gce
$ curl "${KUBERNETES_RELEASE_URL}/v1.6.0-alpha.0.2099+a51bc0dfda11bd/kubernetes.tar.gz" | tar xzf -
$ cd kubernetes
$ cluster/kube-up.sh

[k8s-ci-robot]

Jenkins GCI GCE e2e failed for commit cbee65a. Full PR test history.

The magic incantation to run this job again is @k8s-bot gci gce e2e test this. Please help us cut down flakes by linking to an open flake issue when you hit one in your PR.

[ixdy]

@k8s-bot gci gce e2e test this

[jbeda]

Awesome. LGTM.

/lgtm

[k8s-github-robot]

Automatic merge from submit-queue

[ixdy]

I realized this PR is probably release-note-worthy.

[jbeda]

@ixdy Agree -- release note is worthwhile here.

[arun-gupta]

Yes, @ixdy this would be useful. I was definitely confused by this otherwise!

[ixdy]

@arun-gupta there was supposed to be a more elaborate release note on the original PR, but a rogue space prevented it from being included. :(

[arun-gupta]

@ixdy no worries, glad we found this rather soon!

[k8s-cherrypick-bot]

Commit found in the "release-1.5" branch appears to be this PR. Removing the "cherrypick-candidate" label. If this is an error find help to get your PR picked.