If setting a default route, use default netmask

Without this change, when configuring a default route, it's neccessary
to explicitly provide an ip_netmask of 0.0.0.0/0 - otherwise line 177
errors because there's no route.ip_netmask.

With this change, it's still possible to provide an ip_netmask for a
default route (even though that makes no sense), but if none is provided
The Right Thing is done.

Change-Id: Idac59e571a72cc8d5693f811ec0121273b891d0f
Merge "If setting a default route, use default netmask"