Use YAML_FILENAME_EXTENSIONS for vars files.

The v2 codebase didn't use this previously introduced constant yet.

C.YAML_FILENAME_EXTENSIONS

lib/ansible/vars/__init__.py

@@ -314,7 +314,7 @@ class VariableManager:
             file_name, ext = os.path.splitext(path)
             data = None
             if not ext:
-                for ext in ('', '.yml', '.yaml'):
+                for ext in C.YAML_FILENAME_EXTENSIONS:
                     new_path = path + ext
                     if loader.path_exists(new_path):
                         data = loader.load_from_file(new_path)