Switch py36 jobs to centos-8

We've used bionic for py36 testing, but that's not an official distro
for Victoria. This switches those jobs to centos-8 where it is, which
also has py36 as its default runtime.

Change-Id: I6ad8c6669c2fa3cbdfcaa37f4a159330699f089d
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

zuul.d/jobs.yaml

@@ -150,6 +150,24 @@
     name: openstack-tox-py36
     parent: openstack-tox
     nodeset: ubuntu-bionic
+    branches: ^(stable/(stein|train|ussuri)).*$
+    timeout: 2400
+    description: |
+
+      Run unit tests for an OpenStack Python project under cPython
+      version 3.6.
+
+      Uses tox with the ``py36`` environment.
+    irrelevant-files: *common-irrelevant-files
+    vars:
+      tox_envlist: py36
+      bindep_profile: test py36
+
+- job:
+    name: openstack-tox-py36
+    parent: openstack-tox
+    nodeset: centos-8
+    branches: ^(?!stable/(stein|train|ussuri)).*$
     timeout: 2400
     description: |