Fix error in loader.discover() call

There is no argument "top_level" available. This causes an
error while using the plugin interface.

Change-Id: Id628b68b651144a08ca263dd0dc056987b108e69
Closes-Bug: #1474685
diff --git a/tempest/test_discover/test_discover.py b/tempest/test_discover/test_discover.py
index 86aa855..dac7d91 100644
--- a/tempest/test_discover/test_discover.py
+++ b/tempest/test_discover/test_discover.py
@@ -48,8 +48,8 @@
     for plugin in plugin_load_tests:
         test_dir, top_path = plugin_load_tests[plugin]
         if not pattern:
-            suite.addTests(loader.discover(test_dir, top_level=top_path))
+            suite.addTests(loader.discover(test_dir, top_level_dir=top_path))
         else:
             suite.addTests(loader.discover(test_dir, pattern=pattern,
-                                           top_level=top_path))
+                                           top_level_dir=top_path))
     return suite