ARM Templates - Create or select Service Principal UI page

[101-aks](https://github.com/Azure/azure-quickstart-templates/tree/master/101-aks)

### Issue Details
I am having an impossible time working out how you can create a service principal, much like you see on the UI when deploying the Azure Kubernetes Service from the marketplace.
I am trying to use the 101-aks template to customise my own solution for a turnkey demo/PoC environment and I cant figure out a way to display the create service principal or specify one screen using the existing ARM Templates and UI Definitions. Can someone please suggest how I can do this as part of my workflow. Point me to some documentation or something, so far ive discovered the manual way via the portal, via Powershell & CLI 2.0 but I cant integrate this with the ARM template workflow including the UI Definitions like you see when deploying the Microsoft supported version.

Start with the linked 101-aks template and its ARM template UI Definitions, then compare them with the Azure Marketplace AKS deployment flow described in the issue. Done would be a documented or implemented way to create or select a service principal within that template workflow, rather than using the portal, PowerShell, or CLI separately.

Written by the indexing model from the issue text.