Update docs for Cluster Autoscaler on AKS Engine

[abuccts]

Update docs for Cluster Autoscaler on AKS Engine.

[coveralls]

Coverage decreased (-0.1%) to 34.223% when pulling dad7aaf on xiongyf/ca-docs into 17c7949 on master.

[hzy46]

@abuccts Would you like to add this doc to the openpai handbook? I think we can add a question like How to enable cluster autoscaler in https://openpai.readthedocs.io/en/latest/manual/cluster-admin/installation-faqs-and-troubleshooting.html, and let users refer to https://github.com/microsoft/pai/blob/master/contrib/aks-engine/readme.md.

[abuccts]

Currently there's no way to enable CA in an existing cluster, you need to use aks and deploy with aks-engine from scratch, so this is a standalone part under contrib. It's not applicable for most users, so it might not be suitable for faq?

[hzy46]

Installation FAQ is not just FAQ for the quick-start installation. It could be any questions about the installation. I think it is OK to put this in the FAQ section and tell users who are interested to install the cluster from scratch and try this feature. We may not need to put the whole content. A link to /contrib/aks-engine is enough.