Currently any dhcp agent instance will work as an open resolver. For
deployments using publicly routed addresses for tenant networks, this
allows the agent being abused in dDoS attacks, see [1].
By setting the `--local-service` option dnsmasq will filter DNS queries
and reply only to queries from directly attached networks.
[1] https://bugs.launchpad.net/neutron/+bug/1501206
Closes-Bug: 1501206
Change-Id: I76d810aad2ce0f15a88bd798963012fa0efca74e