Allow the setting of default dnsmasq settings

We can provide default upstream dns servers to dnsmasq.  This will
allow instances's resolv.conf to point to the local name server,
and resolve local domains, as well as resolve upstream domains.
We are no longer using GRE, removed the MTU twiddling.
Merge pull request #89 from blueboxgroup/blueboxgroup/neutron-dnsmasq-config

Allow the setting of default dnsmasq settings