NeutronOVSFirewallDriver guidance and best practices

NeutronOVSFirewallDriver is a TripleO parameter used to implement security groups in an ML2/OVS OpenStack deployment. It may be set to openvswitch or noop (no firewall and no security groups) or iptables_hybrid in an OSP 16.x deployment. The default is iptables_hybrid, but the openvswitch firewall driver uses connection tracking and has hig...