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 add-mtu-and-dhcp-server-to-networks

Change-Id: Id040ce1fb0f889418815568ff6c80402b69bf09e
Add api extension for new network fields.

This uses the existing api extension to implement the actual control
of the fields, but the check is based on a new dummy extension
called os-extended-networks.

Partially-implements blueprint add-mtu-and-dhcp-server-to-networks

Change-Id: I577fe5f6560be50106f345a42a826e97d5e7d64c