Use Python3 yoga unit tests
Use Python3 yoga unit tests

Because the Xena release is not being created for TripleO repos, test
runtime is not updated by automation tools. This change updates
the job template to use the tested runtime for Yoga.

Change-Id: I06108292ef0577e1d71de95c98e3bde8b41ec8c6

openstack/tripleo-puppet-elements,openstack/tripleo-puppet-elements