This option was used to configure firewall driver in the neutron
server's config.
That isn't really used in Neutron since around Newton release as
since Newton agents are reporting to the server what firewall driver
they are using and that is used in neutron ML2 plugin.
Firewall driver can be configured on the agent's side using
``NeutronOVSFirewallDriver`` option which is still here.
Neutron removes usage of this option on the server side with patch
https://review.opendev.org/#/c/726351/
Change-Id: Ifd1527503926b9f86ed567d9396e696d86457715