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.
This is backported to fix an upgrade bug: While ansible 2.3
shouldn't have an issue loading the plugin, upgrading to
Queens would, if the ANSIBLE_ROLE_FETCH_MODE=git-clone is used,
temporarily use ansible to get new roles, including plugins.
That would temporarily load the previous roles with a new
version, and cause failure.
Change-Id: I6e57b64c767bc0c761d61f5f5b07da4f85f1ad08
(cherry picked from commit ad91b1a12b)
Closes-Bug: #1741221
e0ef9e384a