tricircle do not work well with python3-train-jobs

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].
In practice, this generally means adding unit tests for Python 3.7 and
dropping unit tests for Python 3.5.
For tricircle, which already runs py36 unit tests it generally
means switch to use the Zuul template openstack-python3-train-jobs
for Train.
Using this template 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