DHCP agent support for routed VM interfaces

We propose to extend Neutron's DHCP agent code so that it works when
data to/from VMs is being routed by the immediate compute host instead
of being bridged. The routed approach enables various alternative
approaches to providing connectivity between VMs - particularly in
deployments where those VMs only require services at layer 3 (IP) and
above - and it is still possible to implement, through iptables and
route distribution filters, all of the detailed connectivity and
security policies that are implied by any given set of OpenStack's
networking, security group and router configurations.