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

vars_plugins/override_folder.py

@@ -28,6 +28,8 @@ except ImportError:
 
 from ansible.parsing.dataloader import DataLoader
 from ansible.utils.vars import merge_hash
+from ansible import __version__ as ansible_version
+from distutils.version import LooseVersion
 
 
 def vars_files_loading(folder, name, matched=False):
@@ -47,7 +49,7 @@ def vars_files_loading(folder, name, matched=False):
                 files.extend(vars_files_loading(f, name, matched=True))
     return sorted(files)
 
-
+if LooseVersion(ansible_version) < LooseVersion('2.4.0'):
     class VarsModule(object):
         """
         Loads variables for groups and/or hosts