Define py27 assertFoo methods for py26

Frequent issue a test case is failing with py26,
because the used assert method like the assertGreaterEqual
is not defined.

Adding unittest2.TestCase to the base testclass when running
tempest with py26.

Change-Id: I5184fe88485b611c033e81f82177060460da7f1e
diff --git a/tempest/test.py b/tempest/test.py
index 38b9102..457c019 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -17,6 +17,7 @@
 import functools
 import json
 import os
+import sys
 import time
 import urllib
 import uuid
@@ -241,10 +242,23 @@
 
 atexit.register(validate_tearDownClass)
 
-
-class BaseTestCase(testtools.TestCase,
+if sys.version_info >= (2, 7):
+    class BaseDeps(testtools.TestCase,
                    testtools.testcase.WithAttributes,
                    testresources.ResourcedTestCase):
+        pass
+else:
+    # Define asserts for py26
+    import unittest2
+
+    class BaseDeps(testtools.TestCase,
+                   testtools.testcase.WithAttributes,
+                   testresources.ResourcedTestCase,
+                   unittest2.TestCase):
+        pass
+
+
+class BaseTestCase(BaseDeps):
 
     setUpClassCalled = False
     _service = None