Use 0.0.0.0/0 host route as router option

There is a currently a hacky way to get the dhcp-agent to hand out a
gateway route using a host route on the subnet. If you pass a route
that has 0.0.0.0/0 as its destination, dnsmasq will pass it as a static
route. Unfortunately it will also pass the router option if the subnet
has a gateway set. This is inconsistent and precludes users from options
that are available in nova-network like using an external gateway.

This patch fixes things by allowing a 0.0.0.0/0 host route to override
the router setting passed by dnsmasq. This prevents the situation
where dnsmasq hands out two default routes.

Change-Id: Ia70223070bfd437f1c2eb48fde94491d7ee61fcb
Merge "Use 0.0.0.0/0 host route as router option"