Keep using bionic for tox jobs except with Python 3.8
Keep using bionic for tox jobs except with Python 3.8

The default nodeset was just changed to focal [1], which doesn't provide
python3.6 and python3.7 packages.

[1] https://review.opendev.org/c/opendev/base-jobs/+/789098

Change-Id: Ic404d0424978551a99df7e529840e6707ea2df59

openstack/rally,openstack/rally,openstack/rally,openstack/rally