Add Python3 zed unit tests
Add Python3 zed unit tests

Update Zuul job template to recent python versions [1].
python-novaclient 18.0.0 [2] which is listed on upper-constraints [3]
doesn't support python3.6.

[1] https://docs.openstack.org/infra/openstack-zuul-jobs/project-templates.html#project_template-openstack-python3-zed-jobs
[2] https://pypi.org/project/python-novaclient/
[3] https://releases.openstack.org/constraints/upper/master

Change-Id: Ibd4149ce962b6aab084ba3474d22c0bf4e3929cf

openstack/heat-translator,openstack/heat-translator