speed up load.FlowModLoad

This test was responsible for about half the runtime of a full OFTest run.
Reduce the number of iterations to shorten it.
diff --git a/tests/load.py b/tests/load.py
index c2f9013..3295fd5 100644
--- a/tests/load.py
+++ b/tests/load.py
@@ -232,7 +232,7 @@
             request.actions.add(act)
             requests.append(request)
 
-        for i in range(5):
+        for i in range(3):
             logging.info("Iteration %d: delete all flows" % i)
             self.assertEqual(delete_all_flows(self.controller), 0,
                              "Failed to delete all flows")