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