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/tempest b/lib/tempest
index 6ffc927..0d01843 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -387,6 +387,7 @@
     iniset $TEMPEST_CONFIG network default_network "$FIXED_RANGE"
     iniset $TEMPEST_CONFIG network-feature-enabled ipv6 "$IPV6_ENABLED"
     iniset $TEMPEST_CONFIG network-feature-enabled ipv6_subnet_attributes "$IPV6_SUBNET_ATTRIBUTES_ENABLED"
+    iniset $TEMPEST_CONFIG network-feature-enabled port_security $NEUTRON_PORT_SECURITY
 
     # Orchestration Tests
     if is_service_enabled heat; then