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 allocated IP 192.168.0.3 was not used in this guide.
Instead, 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).

Signed-off-by: Vincent Giersch <vincent@giersch.fr>