Tests: Add info to assert_called failure message

This provides enough info to understand why this assertion
is failing without having to attach a debugger.

Change-Id: Ib042039d7589e7c86502ac15773fce995ff64bb9

cinderclient/tests/unit/fakes.py

@@ -66,7 +66,10 @@ class FakeClient(object):
             expected + called)
 
         if body is not None:
-            assert self.client.callstack[pos][2] == body
+            actual_body = self.client.callstack[pos][2]
+            assert actual_body == body, ("body mismatch. expected:\n" +
+                                         str(body) + "\n" +
+                                         "actual:\n" + str(actual_body))
 
         if partial_body is not None:
             try: