Expanded VPN Gateway interoperability

We’re expanding support for the VPN Gateway to allow [Azure Stack](https://azure.microsoft.com/en-us/overview/azure-stack/) users greater flexibility with their settings. Once available, this will allow users to configure their own settings so that they can establish a VPN tunnel with the older devices they have in their data centre, without requiring them to upgrade these devices. This update applies primarily to Azure Stack users. [Learn more](https://docs.microsoft.com/en-us/azure/azure-stack/user/azure-stack-network-overview) about Azure Stack networking. [See more](https://docs.microsoft.com/en-us/azure/azure-stack/azure-stack-key-features) about Azure Stack roles and responsibilities. * Features