[dev][goal][python3][qa][devstack][ptl] changing devstack's python 3 behavior

Today devstack requires each project to explicitly indicate that it can be installed under python 3, even when devstack itself is running with python 3 enabled. As part of the python3-first goal, I have proposed a change to devstack to modify that behavior [1]. With the change in place, when devstack runs with python3 enabled all services are installed under python 3, unless explicitly listed as not supporting python 3. If your project has a devstack plugin or runs integration or functional test jobs that use devstack, please test your project with the patch (you can submit a trivial change to your project and use Depends-On to pull in the devstack change). [1] https://review.openstack.org/#/c/622415/ -- Doug