Small private network guide update

- Remove mix of 192.168.0.2 and 192.168.0.3 IPs for gateway address:
it looks like the OpenStack allocated IP 192.168.0.3 was not used in
this guide. Instead, in this update, force allocated IP to 192.168.0.3
when creating neutron port and use it as the gateway IP.
- When using neutron CLI to create subnet, add the configured gateway
IP as the subnet gateway (as done in Horizon).