Send the router its default gateway IP

Each router may have multiple public IPs, but we only want it to use a single
default gateway. We work out what that value should be in the rug, and send
it to the appliance so the appliance does not have to figure it out for
itself.

Change-Id: Ie8ae8a866ee46ed9b0b3d5ef79c2a4fadf1b7f0a