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/stackrc b/stackrc
index c419ef4..ea8b044 100644
--- a/stackrc
+++ b/stackrc
@@ -778,6 +778,9 @@
 
 HOST_IPV6=$(get_default_host_ip "" "" "$HOST_IP_IFACE" "$HOST_IPV6" "inet6")
 
+# Whether or not the port_security extension should be enabled for Neutron.
+NEUTRON_PORT_SECURITY=$(trueorfalse True NEUTRON_PORT_SECURITY)
+
 # SERVICE IP version
 # This is the IP version that services should be listening on, as well
 # as using to register their endpoints with keystone.