Make ansible version configurable

Currently the default ansible version is selected by the version of
zuul itself. However we want to make this configurable per deployment
(zuul.conf), tenant and job.

Change-Id: Iccbb124ac7f7a8260c730fbc109ccfc1dec09f8b

doc/source/user/config.rst

@@ -895,6 +895,21 @@ Here is an example of two job definitions:
 
          run: playbooks/job-playbook.yaml
 
+   .. attr:: ansible-version
+
+      The ansible version to use for all playbooks of the job. This can be
+      defined at the following layers of configuration where the first match
+      takes precedence:
+
+      * :attr:`job.ansible-version`
+      * :attr:`tenant.default-ansible-version`
+      * :attr:`scheduler.default_ansible_version`
+      * Zuul default version
+
+      The supported ansible versions are:
+
+      .. program-output:: zuul-manage-ansible -l
+
    .. attr:: roles
 
       A list of Ansible roles to prepare for the job.  Because a job