Make possible to use ansible 2.4
Make possible to use ansible 2.4

Ansible 2.4 has changed the interface for var plugins.
Our vars plugin shouldn't be used in ansible 2.4, because the
features we were looking for are now standard.

However the plugin is still shipped. So we need to ensure
this vars plugin doesn't get loaded on ansible 2.4.

Ansible will not load a vars plugin in its plugin location
if no VarsModule is defined. This commit abuses this
mechanism to conditionally load the vars module depending on
the version, at the expense of throwing a warning in
Ansible 2.4.

Change-Id: I6e57b64c767bc0c761d61f5f5b07da4f85f1ad08

openstack/openstack-ansible-plugins,openstack/openstack-ansible-plugins