Adding an option of "Soft Restart" in neutron agent along with ovs-firewall-driver

While the blueprint of "ovs-firewall-driver" is being developed, a new concern comes up. When an ovs agent (or an ml2 agent with ovs) restarts, if it cleans up all the pre-existing OpenFlow flows and recreates them, it could be problematic because it would cause indefinite network hiccups on VMs' traffic. So we need to add an option of soft-restart to an ovs agent, which allows to keep (or not touch) the pre-existing OpenFlow flows and possibly fill the missing OpenFlow flows only. Maybe it would be also better to delete undesired OpenFlow flows as well. Basically, "soft-restart" would be fixing flows without touching legitimate flows.