Make validate atcs optional

Without this patch, a change of membership of any ATC could break
the lint jobs.

This is a problem, as the change of membership can happen anytime.

This fixes it by creating a new job to show whether atc validation
fails or not, as non voting. This brings increased visibility to
the eventual membership problems, compared to having them listed
in stdout like we used to do in the past.

Change-Id: I5cb3dee7809d80369eaf8c955695b1389f5eb5d4

.zuul.yaml

@@ -1,9 +1,21 @@
+- job:
+    name: openstack-tox-validate_atcs
+    parent: openstack-tox-linters
+    description: |
+      Run the ATC checking to show if the current ATCs are still valid.
+      Foundation membership can change anytime, and we don't want our linter
+      jobs to fail due to this, but we want to be informed at all times.
+    vars:
+      tox_envlist: validate_atcs
+
 - project:
     templates:
       - build-openstack-docs-pti
       - openstack-python3-ussuri-jobs
     check:
       jobs:
+        - openstack-tox-validate_atcs:
+            voting: false
         - openstack-tox-linters:
             required-projects:
               - openstack/project-config

tox.ini

@@ -36,7 +36,10 @@ commands =
   flake8
   whereto {toxinidir}/doc/source/_extra/.htaccess {toxinidir}/doc/test/redirect-tests.txt
   {toxinidir}/tools/validate_repositories.py
-  {toxinidir}/tools/validate_atcs.py
+
+[testenv:validate_atcs]
+basepython = python3
+commands = {toxinidir}/tools/validate_atcs.py
 
 [testenv:pep8]
 basepython = python3