feat(ext): Add choco extension with param support

[sylus]

Is this a request for help?: Adds a Windows extension example; in this case an extension which installs packages passed as parameters via the Choco package manager. Very similar to how the one that works in DevTestLabs.

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

Links to P.R. #2707

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What version of acs-engine?: v0.15.2

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Orchestrator and version (e.g. Kubernetes, DC/OS, Swarm) Kubernetes

What happened: Installs specified choco package.

What you expected to happen: Installs specified choco package.

How to reproduce it (as minimally and precisely as possible): Run cluster definiton then az group deployment.

Anything else we need to know: Confirmed and tested with microsoft-build-tools and it provisioned by Windows with the package.

I tested this on a generated cluster definition on top of v0.15.2

I did have to fix an entry in azuredeploy.json to get everything to work so think still need a fix in code.

"name": "[concat(variables('windowspool1VMNamePrefix'), copyIndex(variables('windowspool1Offset')), '/cse')]",

Should be this instead:

 "name": "[concat(variables('windowspool1VMNamePrefix'), copyIndex(variables('windowspool1Offset')),'/cse', copyIndex(variables('windowspool1Offset')))]",