Replace `CONF.network-feature-enabled.port_security` with code obtaining information.

As seen in a patch introducing option `network-feature-enabled.port_security` [1] - option only being set manually according to neutron extension enabled/disabled in ml2.conf, therefore such information can be spared and automated in neutron whitebox plugin (instead of maintained in CI).

More details in parent story.

[1] 234995: Add scenario tests for ml2 port-security plugin | https://review.gerrithub.io/c/cisco-openstack/tempest/+/234995