Add whereto test as optional step in doc build

If a project has whereto installed in its venv tox environment, use it
to run tests for the redirects in the .htaccess file that is part of
its documentation.

This is a port of https://review.openstack.org/#/c/502089 for zuulv3.

Change-Id: I9cfcc5f81487708e58022b83e24974fef653779f
Co-Authored-By: Doug Hellmann <doug@doughellmann.com>

playbooks/openstack-doc-build/run.yaml

@@ -0,0 +1,17 @@
+- hosts: all
+  roles:
+    - tox
+
+  post_tasks:
+
+    - name: check for whereto
+      stat:
+        path: "{{ zuul_work_dir }}/.tox/{{ tox_envlist }}/bin/whereto"
+      register: whereto
+
+    - name: Run tox again for whereto
+      include_role:
+        name: tox
+      vars:
+        tox_extra_args: whereto doc/source/_extra/.htaccess doc/test/redirect-tests.txt
+      when: whereto.stat.exists

zuul.yaml

@@ -58,6 +58,7 @@
 - job:
     name: openstack-doc-build
     parent: tox-docs
+    run: playbooks/openstack-doc-build/run
     post-run: playbooks/openstack-doc-build/post
     required-projects:
       - name: openstack/requirements