Server should set the gateway from the ip (CIDR) if the user don't provide the gateway.

[hwchiu]

For the server side, we should calculate the gateway from the IP (CIDR) and use it as the gateway if user don't provide the Gateway.

For the client side, we only accept the following combination

1. valid IP + valid Gateway
2. valid IP + empty Gateway
3. empty IP + empty Gateway (Won't set ip address in this case)

[hwchiu]

The gateway should only exist if we want the user set the static route.
The combination will be

1. empty IP + empty static-route
2. valid IP
3. valid IP + valid static-rotue

A static-route is CIDR+IP

[John-Lin]

#80