Comment 19 for bug 1468236

@Eugene Nikanorov, 'similar to DVR' is just means running dhcp-agent on every compute node, but it has below benefit :
1. it will simplify the dhcp-agent schedule in neutron-server, when we create a VM, neutron-server just send the RPC message by port host id
2. the dhcp-agent running on a compute node just serve the VMs on this compute node, if this dhcp-agent down, it just affect the VMs running on this node
3. in large-scale, even if there are a large numbers VM in this cloud, but for a compute node, there is a certain amount of VMs on a compute node
4. it will not complicate ovs-agent the code, maybe it just need to add some flow