rds-update-rdsh-collection - doesn't correctly use an existing vnet in different ResourceGroup

Existing vnet doesn't always work, since it assumes the vnet is in the same ResourceGroup as the current deployment, which isn't possible with Azure Marketplace deployment and doesn't work with typical ExpressRoute setups (where the ExpressRoute is all by itself in a resource group).

Behavior should model the one here:
https://github.com/Azure/azure-quickstart-templates/tree/master/101-vm-from-user-image
where...
- there is an "existingvnet.json" file
- there is a "newvnet.json" file
- the variable for which .json file to use is a concat of ["new/existing" in the variable] and "vnet.json"
- the network interface "dependsOn" this file
- the Subnet for the network interface is the output of this

Start by locating the deployment templates and the existingvnet.json and newvnet.json files described in the issue. Trace how the variable selects the template, then verify that the network interface depends on the selected file and that its subnet comes from that file's output; done means existing VNets can reside in a different ResourceGroup.

Written by the indexing model from the issue text.