Remove ER/VPN GW subnet input requirements for vWAN deployments

[daltondhcp]

Describe the bug
When deploying the RI with vWAN network topology along with VPN/ER gateway, gateway subnet CIDR validation fails regardless of input.
Since the vWAN hub assigns subnets automatically to various gateways from the managed hub address space, gateway subnet input should not be asked for at all in those scenarios.

Note that the subnet input requirement was introduced in #613, but the CIDR validation issue helped us to surface this 😃

Steps to reproduce

1. Launch Azure Landing Zone accelerator, and complete sections until arrive to the Network topology and connectivity section.
2. Select Virtual WAN (Microsoft Managed)
3. Select to deploy VPN/ExpressRoute gateway or VPN gateway
4. Notice how the subnet validation fails regardless of input

Screenshots