Adds tox-molecule job

This adds job definition for running Ansible Molecule testing framework
from inside tox.

This would ease use of molecule testing for any projects that
want to use by avoiding the need to duplicate the setup code across
each repository.

It uses tox `molecule` environment.

Change-Id: I8df4df5448a78b9e21f45fac12d534eb84b29d3d
Needed-By: https://review.opendev.org/#/c/663599/
Needed-By: https://review.opendev.org/#/c/651772/
Needed-By: https://review.opendev.org/#/c/663336/

doc/source/python-jobs.rst

@@ -9,6 +9,7 @@ Python Jobs
 .. zuul:autojob:: tox-py37
 .. zuul:autojob:: tox-docs
 .. zuul:autojob:: tox-linters
+.. zuul:autojob:: tox-molecule
 .. zuul:autojob:: tox-pep8
 .. zuul:autojob:: tox-cover
 .. zuul:autojob:: tox-bashate

playbooks/tox/pre-molecule.yaml

@@ -0,0 +1,3 @@
+- hosts: all
+  roles:
+    - install-docker

zuul.yaml

@@ -212,6 +212,17 @@
     vars:
       tox_envlist: linters
 
+- job:
+    name: tox-molecule
+    parent: tox
+    pre-run: playbooks/tox/pre-molecule.yaml
+    description: |
+      Runs Ansible molecule testing on a system where docker is available.
+
+      Uses tox with the ``molecule`` environment.
+    vars:
+      tox_envlist: molecule
+
 - job:
     name: tox-pep8
     parent: tox