Change tox.ini so that all jobs are python3, and add explicit
unit/functional testenvs for the two ussuri python runtimes
(3.6 and 3.7).
Remove py2 jobs from .zuul.yaml and redefine the periodic "tips"
jobs so that they are testing with the ussuri python runtimes.
Added nodeset: ubuntu-bionic to the abstract base jobs so that
py3.7 is available.
Co-authored-by: Ghanshyam Mann <gmann@ghanshyammann.com>
Co-authored-by: Brian Rosmaita <rosmaita.fossdev@gmail.com>
Closes-bug: #1853693
Change-Id: I15467c34fc8466fa1724c5401dc879ae77c1922d
40bd312410