add python 3.6 unit test job
add python 3.6 unit test job

This is a mechanically generated patch to add a unit test job running
under Python 3.6 as part of the python3-first goal.

See the python3-first goal document for details:
https://governance.openstack.org/tc/goals/stein/python3-first.html

Change-Id: Ib7b0c2be73232a35ce8d79380122e26b6af03b0a
Story: #2002586

openstack/python-mistralclient,openstack/python-mistralclient