basic support for using external static dhcp

This blueprint will try to give support instance using external static dhcp, instead of using neutron dhcp.
Currently neutron allow booting instance on external network, and with setting disable dhcp on subnet, instance get can IP from external dhcp server. But for neutron doesn't support that yet, so we have to do some handcrafts like, updating port, updating security group on compute node, to make instance's networking work.
The basic support this blueprint will try to work on, is making sure instance's networking will work
without additional handcrafts.