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: I5f9cda5cef438a1eef67cdcaddae8eeb24910cca
Story: #2002586
Task: #24322

openstack/oslo.service