Allow dhcp_server to be set from new field

Now that dhcp_server and gateway can be different, update dnsmasq
to hand out the gateway if we have a single ip address and they
don't match.

Partially-implements blueprint better-support-for-multiple-networks

Change-Id: Id040ce1fb0f889418815568ff6c80402b69bf09e
Merge "Allow dhcp_server to be set from new field"