Fix zuul-stream-functional tests
The pre.yaml playbook for the zuul-stream-functional tests copies
the Ansible inventory.yaml file from the executor to the test
"controller" node. The controller then runs the specified version
of Ansible 2.x against the other nodes. This fails because the
executor version of inventory.yaml contains "auto" for the Ansible
python interpreter which is valid under the version of Ansible used
on the executor, but on the controller node, which runs the older
versions of Ansible, this is *not* a valid value. Thus it fails.
This change forces the executor to use the version of Ansible being
tested on the controller so that the inventory.yaml will be correct.
Also, Ansible 2.8 now throws a FileNotFoundError exception instead
of OSError when a referenced file is not found.