MLZ & Tier3: add route table to Gateway Subnet

[jamasten]

Add a route table to the GatewaySubnet so that on premises traffic can route to spoke and tier 3 subnets appropriately. This change will ensure that if a Virtual Network Gateway or S2S VPN connections are created later, incoming traffic will route through the Azure Firewall before going to the spoke VNETs to avoid inadvertent firewall bypass.