Use zuul template for python3 test runtime

This goal is to implement the process set out in the 2018-10-24 Python
Update Process TC resolution[1], for the Train cycle to ensure unit
testing is in place for all of the Tested Runtimes for Train[2].
Using the Zuul template for Train will ensure that all projects that
support Python3 will be tested against the agreed runtime versions,
and make it easier to update them in future.

[1]https://governance.openstack.org/tc/resolutions/20181024-python-update-process.html
[2]https://governance.openstack.org/tc/reference/runtimes/train.html

Change-Id: I406a4f1ecbff5b6c8e18ff1b93a3b2ceef49343a