Migrate Python 3.6/7 jobs to Python 3.8
Migrate Python 3.6/7 jobs to Python 3.8

Support for Python 3.6 and 3.7 is being removed globally. The current
latest release of python-novaclient (18.0.0) does not support these two
versions. The next python-neutronclient release does not, either.

This migrates Python 3.6/7 jobs to Python 3.8 because 3.8 is now
the minimum supported version.

This also replaces Python3 yoga unit tests by zena unit tests.

Depends-on: https://review.opendev.org/843115
Change-Id: I03957cf4bd0a96cb1d07e80727c184854b869fc3

openstack/cliff,openstack/cliff