Add VMSS to an existing vnet/subnet

[301-custom-images-at-scale](https://github.com/Azure/azure-quickstart-templates/tree/master/301-custom-images-at-scale)

### Issue Details
I am trying to get VMSS deployed to an existing vnet/subnet. Could you please let me know what need to be changed in the template?

### Repro steps (*if necessary, delete otherwise*)
1. I changed variables when template was imported in Azure Portal
2. Tried to deploy
3. Deployment failed with "Server Internal Error"
4.
5.

Start with the 301-custom-images-at-scale template linked in the issue and reproduce the deployment after selecting an existing VNet and subnet in the Azure Portal. Compare the template's networking inputs with the supplied existing resources and use the deployment error details to locate the failure. Done means VMSS deploys successfully into the existing VNet/subnet.

Written by the indexing model from the issue text.