Add documentation and validation for "Location" field

[jackfrancis]

Let's iterate over the existing AKS Engine documentation and ensure that we have sufficient doc coverage of the fact that the api model "location" property is required for non-public clouds.

Also, let's add a validation stdout warning for api model generation/deployment when the "location" property is empty:

WARNING: No "location" was specified, AKS Engine will generate an ARM template configuration valid for public cloud only.