As part of the python 3 first effort [1], this patch adds UT support for
python 3.6 by:
- Updating the zuul.yaml to include the py36 template and jobs.
- Adding a py36-dev target to tox.ini for local testing.
- Updating the upper constraints to use master rather than stable/rocky.
- Updating the version of eventlet used as previous 2x.x versions had
a python 3 bug for monkey patching.
- Consolidating the eventlet monkey patching in the code.
[1] https://governance.openstack.org/tc/goals/stein/python3-first.html
Change-Id: I3c11ac9dff9a2e25b33e8310929c961ffd0bcc52
822f7c4cf7