Changes for Ansible 2.4

Squashed changes:
   - Use 'inventory' instead of 'hostfile' in ansible.cfg.

     'hostfile' is deprecated.

   - Use 'os.environ.copy()' in zuul_return.py since this causes 2.4 to
     throw an exception now deep within module.exit_json().

Change-Id: I0a52c9e169a54d24a7b361010045fb10211418b7
diff --git a/playbooks/zuul-stream/templates/ansible.cfg.j2 b/playbooks/zuul-stream/templates/ansible.cfg.j2
index 24f459e..41ffc0c 100644
--- a/playbooks/zuul-stream/templates/ansible.cfg.j2
+++ b/playbooks/zuul-stream/templates/ansible.cfg.j2
@@ -1,5 +1,5 @@
 [defaults]
-hostfile = {{ ansible_user_dir }}/inventory.yaml
+inventory = {{ ansible_user_dir }}/inventory.yaml
 gathering = smart
 gather_subset = !all
 lookup_plugins = {{ ansible_user_dir }}/src/git.openstack.org/openstack-infra/zuul/zuul/ansible/lookup