Fix nox job python2.7 support

Virtualenv dropped python2.7 support. When we install latest nox it also
installs latest virtualenv. Then when we try to run python2.7 jobs which
will create a python2.7 venv things fail with:

  RuntimeError: failed to find interpreter for Builtin discover of python_spec='python2.7'

The underlying issue is that we are getting a too new virtualenv. Fix
this by adding a pre run step to the python2.7 jobs that downgrades to a
version of virtualenv that should support python2.7 environments.

Change-Id: I0ed9915b25c752cdaae4df19c9a0e0dc1aafd3d7

playbooks/nox/pre27.yaml

@@ -0,0 +1,6 @@
+- hosts: all
+  tasks:
+    - name: Downgrade virtualenv for python2.7 support
+      # Assume pip is installed in the same relative path as nox
+      # and use that to downgrade virtualenv to a version python2.7 supports.
+      command: "{{ nox_executable | dirname }}/pip install -U virtualenv<20.22.0"

zuul.d/python-jobs.yaml

@@ -403,6 +403,7 @@
       Run unit tests for a Python project under cPython version 2.7.
 
       Uses nox with the ``test`` keyword forcing python 2.7.
+    pre-run: playbooks/nox/pre27.yaml
     vars:
       nox_keyword: tests
       nox_force_python: "2.7"