Zuul runs tests in py36 only

Python 3.6 is the oldest version of python
supported by the package, and newer, supported,
minor versions do not break any substantial functionality
by design, while introducing new features.

Therefore we do not need to run unit tests for
each minor version. As all possible issues affecting
versions >3.6 would necessarily be related to features
introduced in versions >3.6.

Wallaby Zuul test template was removed from layout.yaml.
The pep8 test environment was folded into linters in tox.ini.
Zuul is now explicitly running tests only in one, oldest supported,
python environment, Python 3.6.

PEP387: https://www.python.org/dev/peps/pep-0387/
https://docs.python.org/3/whatsnew/3.7.html#porting-to-python-37
https://docs.python.org/3/whatsnew/3.8.html#porting-to-python-3-8

Signed-off-by: Jiri Podivin <jpodivin@redhat.com>
Change-Id: If8e31e72b215dcdb0db897b03403e8d28a2545b3

tox.ini

@@ -93,6 +93,7 @@ deps =
     -r {toxinidir}/molecule-requirements.txt
 commands =
     python '{toxinidir}/tools/validate-files.py' .
+    {[testenv:pep8]commands}
     {[testenv:ansible-lint]commands}
     {[testenv:yamllint]commands}
     {[testenv:bashate]commands}

zuul.d/layout.yaml

@@ -2,7 +2,6 @@
 - project:
     templates:
       - tripleo-multinode-container-minimal-pipeline
-      - openstack-python3-wallaby-jobs
       - tripleo-validations-molecule-jobs
       - release-notes-jobs-python3
     check:
@@ -26,12 +25,14 @@
               - tripleo-ci-centos-8-content-provider
             files:
               - ^roles/ceph.*$
+        - openstack-tox-py36
     gate:
       jobs:
         - openstack-tox-linters
         - openstack-tox-docs: *tripleo-docs
         - tripleo-ci-centos-8-content-provider
         - tripleo-ci-centos-8-scenario004-standalone: *scenario004
+        - openstack-tox-py36
 
     promote:
       jobs: