The neutron-gateway uses the firewall driver just as other nodes
do when running neutron-openvswitch-agent. It is currently
hardcoded to the deprecated iptables_hybrid driver. This patch
allows the driver to be changed to openvswitch same as with the
neutron-openvswitch driver with a firewall-driver config option
that defaults to iptables_hybrid so as to maintain backwards
compatibility.
Change-Id: I4f5482425c91b5ad556c384abba7c27137c1948f
3de85d46c1