Add fwaas extension in l3 agent when neutron fwaas is enabled

Change-Id: I3b3ad184dea6b93f21c8778386c04a93b38bc2af
Closes-Bug: #1683802

ansible/roles/neutron/templates/l3_agent.ini.j2

@@ -13,4 +13,7 @@ agent_mode = legacy
 [fwaas]
 driver = neutron_fwaas.services.firewall.drivers.linux.iptables_fwaas.IptablesFwaasDriver
 enabled = True
+
+[agent]
+extensions = fwaas
 {% endif %}