Use immutable arg rather mutable arg

Passing mutable objects as default args is a known Python pitfall.
We'd better avoid this. This commit changes mutable default args with
None, then use 'arg = arg or []'.

Change-Id: I52a37bbc129fab40a99c919a837147d663941dce

keystoneclient/tests/fakes.py

@@ -21,7 +21,9 @@ places where actual behavior differs from the spec.
 from keystoneclient import access
 
 
-def assert_has_keys(dict, required=[], optional=[]):
+def assert_has_keys(dict, required=None, optional=None):
+    required = required or []
+    optional = optional or []
     keys = dict.keys()
     for k in required:
         try: