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: I02c96a9327e2ccd6c4d5f3f175bb9f507bb026d7

openstack/freezer-api,openstack/freezer-api,openstack/freezer-api,openstack/freezer-api