add group decorator and smoke test group

The smoke group is intended to be very fast while covering most important
features.
diff --git a/tests/port_stats.py b/tests/port_stats.py
index 89b2cf6..8d39d6f 100644
--- a/tests/port_stats.py
+++ b/tests/port_stats.py
@@ -129,6 +129,7 @@
     obj.assertTrue(all_packets_received,
                    "Packet received does not match number sent")
 
+@group('smoke')
 class SingleFlowStats(base_tests.SimpleDataPlane):
     """
     Verify flow stats are properly retrieved.