custom data in addons is not parsed (gunzip+base64 decoded) properly

[sgoings]

Is this a request for help?: Help, in a way, yes.

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

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What version of aks-engine?: 0.28.1

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Kubernetes version: any

What happened: Supply an addon manifest through aks-engine's data apimodel field for addon config in [gzip+base64 OR base64] format. My scenario is to provide a specific manifest for ip-masq-agent, but it probably applies to any addon of this type. The content on disk of the resultant addons config file is the same as the input format (no [gunzip+base64 OR base64] decoding has happened).

What you expected to happen: Content to be decoded appropriately and placed as a valid yaml manifest in the appropriate file in /etc/kubernetes/addons/

Anything else we need to know: Documentation in the section: https://github.com/Azure/aks-engine/blob/master/docs/topics/clusterdefinitions.md#addons points users to both: "gzip+base64 encode" as well as "base64" encode their addon manifests when putting into the aks-engine api model. Neither is correct at the current time.

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