Add template to deploy Windows VMSS into existing VNet

[gbowerman]

Best Practice Checklist

Check these items before submitting a PR... See the Contribution Guide for the full detail: https://github.com/Azure/azure-quickstart-templates/blob/master/1-CONTRIBUTION-GUIDE/README.md

1. uri's compatible with all clouds (Stack, China, Government)
2. Staged artifacts use _artifactsLocation & _artifactsLocationSasToken
3. Use resourceGroup().location for resource locations
4. Folder names for artifacts (nestedtemplates, scripts, DSC)
5. Use literal values for apiVersion (no variables)
6. Parameter files (GEN-UNIQUE for value generation and no "changemeplease" values
7. $schema and other uris use https
8. Use uniqueString() whenever possible to generate names for resources. While this is not required, it's one of the most common failure points in a deployment.
9. Update the metadata.json with the current date

For details: https://github.com/Azure/azure-quickstart-templates/blob/master/1-CONTRIBUTION-GUIDE/bp-checklist.md

• - Please check this box once you've submitted the PR if you've read through the Contribution Guide and best practices checklist.

Changelog