Improve error info in assertEmpty and assertNotEmpty

When assertEmpty fails, it will print:
        raise mismatch_error
    MismatchError: 0 != 5

this is to improve the error info as:
        raise mismatch_error
    MismatchError: 0 != 1: list is not empty: [u'1b95a8ea7e124a76bfadf6ce1906aa27']

and when assertNotEmpty fails, it will print:
        raise self.failureException(msg)
    AssertionError: 0 not greater than 0

this is to improve the error info as:
        raise self.failureException(msg)
    AssertionError: list is empty.

Change-Id: Icdf69b596fa135387721c49df164a68799ab9a70
diff --git a/tempest/test.py b/tempest/test.py
index 970e97c..52994ac 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -643,7 +643,11 @@
             cred_provider, networks_client, CONF.compute.fixed_network_name)
 
     def assertEmpty(self, list, msg=None):
+        if msg is None:
+            msg = "list is not empty: %s" % list
         self.assertEqual(0, len(list), msg)
 
     def assertNotEmpty(self, list, msg=None):
+        if msg is None:
+            msg = "list is empty."
         self.assertGreater(len(list), 0, msg)