Fix example for running individual tests
Fix example for running individual tests

The ':' syntax doesn't seem to work anymore. Additionally fix the "tox" call to
not try to run it from the current working directory. It's usually installed
somewhere in $PATH.

Closes-Bug: #1334134
Change-Id: Ie608cee2ac131941d6642e2560a84db55415654b

openstack/neutron-fwaas,dougwig/x-neutron-fwaas,citrix-openstack-build/neutron-fwaas,gaolichuang/neutron-fwaas,openstack/neutron-fwaas,gaolichuang/neutron-fwaas