Prevent openstack-tox-py27 from running on focal

While we don't want to be using bionic across all stable branches, focal
doesn't have py27 available. Use xenial for pre-stein, and bionic for
stein and later.

Change-Id: I206b95ee9240345e91229921b6cf14e13f36d254
Related-Change: I7c597c4aa5aeb3b1bb7987efb4afd0c5c15d86e6

zuul.d/jobs.yaml

@@ -115,11 +115,13 @@
 - job:
     name: openstack-tox-py27
     parent: openstack-tox
+    nodeset: ubuntu-bionic
     timeout: 2400
     description: |
       Run unit tests for an OpenStack Python project under cPython version 2.7.
 
       Uses tox with the ``py27`` environment.
+    branches: ^(?!stable/(ocata|pike|queens|rocky)).*$
     irrelevant-files: &common-irrelevant-files
       - ^.*\.rst$
       - ^api-ref/.*$
@@ -130,6 +132,29 @@
       tox_envlist: py27
       bindep_profile: test py27
 
+- job:
+    name: openstack-tox-py27
+    parent: openstack-tox
+    nodeset: ubuntu-xenial
+    timeout: 2400
+    description: |
+      Run unit tests for an OpenStack Python project under cPython version 2.7.
+
+      Uses tox with the ``py27`` environment.
+
+      This job runs on Xenial for stable/ocata, pike, queens and rocky. This
+      job is prepared to make sure all stable branches before stable/stein will
+      keep running on xenial.
+    branches:
+      - stable/ocata
+      - stable/pike
+      - stable/queens
+      - stable/rocky
+    irrelevant-files: *common-irrelevant-files
+    vars:
+      tox_envlist: py27
+      bindep_profile: test py27
+
 - job:
     name: openstack-tox-py35
     parent: openstack-tox