Avoid using built-in function name

Avoid using built-in function name. Here filter is a built-in
function name. As per python standard we should not use it.

Change-Id: I6a005ca55d42dd41f9ac0b482bee056a23671190

tacker/auth.py

@@ -72,6 +72,6 @@ def pipeline_factory(loader, global_conf, **local_conf):
     filters = [loader.get_filter(n) for n in pipeline[:-1]]
     app = loader.get_app(pipeline[-1])
     filters.reverse()
-    for filter in filters:
-        app = filter(app)
+    for f in filters:
+        app = f(app)
     return app