tempest: set network-feature-enabled.port_security

Sets the port_security feature flag in tempest.conf
if the port_security extension is enabled, which it's not
by default in neutron but is set by default in devstack.

This adds global variable for setting the port_security
extension in ml2.conf and in tempest.conf so we only have
to set this in one place.

Depends-On: I1efd5c838aa0d73cc6e8864e3041eea25850198d

Change-Id: I6334b200e42edd785f74cfb41520627393039619
Related-Bug: #1624082
diff --git a/lib/neutron b/lib/neutron
index 53fc5fc..415344e 100644
--- a/lib/neutron
+++ b/lib/neutron
@@ -162,7 +162,9 @@
         iniset $NEUTRON_CORE_PLUGIN_CONF ml2 type_drivers vxlan
         iniset $NEUTRON_CORE_PLUGIN_CONF ml2 mechanism_drivers openvswitch,linuxbridge
         iniset $NEUTRON_CORE_PLUGIN_CONF ml2_type_vxlan vni_ranges 1001:2000
-        iniset $NEUTRON_CORE_PLUGIN_CONF ml2 extension_drivers port_security
+        if [[ "$NEUTRON_PORT_SECURITY" = "True" ]]; then
+            iniset $NEUTRON_CORE_PLUGIN_CONF ml2 extension_drivers port_security
+        fi
     fi
 
     # Neutron OVS or LB agent