Use unittest2.TestSuite with py26

The tempest master branch with tox -efull with py26 command,
produces a lot of tierDownClass failures,
because it tries to tearDown an already tearDowned class.

By switching to unittest2.TestSuite the issue is solved.

Change-Id: I7a5d21af62bafb13cdc8a0dcc20ed3f742aa214f
diff --git a/tempest/test_discover/test_discover.py b/tempest/test_discover/test_discover.py
index 87fdbdc..3fbb8e1 100644
--- a/tempest/test_discover/test_discover.py
+++ b/tempest/test_discover/test_discover.py
@@ -13,7 +13,12 @@
 #    under the License.
 
 import os
-import unittest
+import sys
+
+if sys.version_info >= (2, 7):
+    import unittest
+else:
+    import unittest2 as unittest
 
 
 def load_tests(loader, tests, pattern):