Fix Mutable default argument

Python’s default arguments are evaluated once when
the function is defined, not each time the function is called.

This means that if you use a mutable default argument and
mutate it, you will and have mutated that object for all
future calls to the function as well.

more details about this wrong usage here:
http://docs.python-guide.org/en/latest/writing/gotchas/#mutable-default-arguments
Closes-Bug: #1534974

Change-Id: Ia87abe0d020d7840dbf5f8a3063f057bab92c2ba

manilaclient/tests/unit/fakes.py

@@ -21,7 +21,11 @@ places where actual behavior differs from the spec.
 from __future__ import print_function
 
 
-def assert_has_keys(dictonary, required=[], optional=[]):
+def assert_has_keys(dictonary, required=None, optional=None):
+    if required is None:
+        required = []
+    if optional is None:
+        optional = []
     for k in required:
         try:
             assert k in dictonary

manilaclient/tests/unit/v2/test_shell.py

@@ -800,7 +800,9 @@ class ShellTest(test_utils.TestCase):
     def test_extract_metadata(self):
         # mimic the result of argparse's parse_args() method
         class Arguments:
-            def __init__(self, metadata=[]):
+            def __init__(self, metadata=None):
+                if metadata is None:
+                    metadata = []
                 self.metadata = metadata
 
         inputs = [