Support custom URLs for Windows node binaries

[PatrickLang]

Is this an ISSUE or FEATURE REQUEST? (choose one):
Feature Request

What version of acs-engine?:
v0.13.1

We'd like to be able to test daily builds using ACS-engine. Ideally this would take in a URL to a ZIP file containing kubelet.exe, kube-proxy.exe and wincni.exe. These will be put into the custom windows image that's deployed as the cluster is built out.

This is intended to support testing new Kubernetes releases as well as more frequent PR testing triggered by prow from master k8s builds.

[PatrickLang]

here's the code that downloads and extracts the ZIP

acs-engine/parts/k8s/kuberneteswindowssetup.ps1

Line 117 in c51a016

Invoke-WebRequest -Uri $global:KubeBinariesSASURL -OutFile $zipfile

[PatrickLang]

wincni is separated already:

acs-engine/parts/k8s/kuberneteswindowssetup.ps1

Line 139 in c51a016

Invoke-WebRequest -Uri $url -OutFile $wincniFile