rds-deployment-existing-ad template should allow specify a VNET from a different RG

[mmarch]

Need a parameter to specify which resource group the existing VNET belongs to.

"... the VNET is expected to be in the same RG as the other resources. In most of deployments this is not the case. "

"... current requirement of the Basic RDS Template with Existing AD, is that the VNET resides in the same Resource Group. This limitation came up in earlier discussion and we already identified this as something that we need to change going forward."

[jasonagilbertson]

This issue was moved to Azure/RDS-Templates#46