169163334 - Adds Virtual Network Gateway for VPN

The Virtual Network Gateway is required for OpenVPN connectivity. The
change to the VPC module also adds a subnet which is exclusively used
for the Gateway.

terraform/providers/dev/variables.tf

@@ -31,6 +31,12 @@ variable "networks" {
   }
 }
 
+variable "gateway_subnet" {
+  type    = string
+  default = "10.1.20.0/24"
+}
+
+
 variable "route_tables" {
   description = "Route tables and their default routes"
   type        = map