Fix the import of NotFound exception

It is exported by tempest.lib.exceptions, not tempest.exceptions
(it was originally until 2015).

Change-Id: Ic8ef45bcb9518968d32872a93fc3381004d218ff
diff --git a/barbican_tempest_plugin/tests/scenario/manager.py b/barbican_tempest_plugin/tests/scenario/manager.py
index 8daf090..621ee7c 100644
--- a/barbican_tempest_plugin/tests/scenario/manager.py
+++ b/barbican_tempest_plugin/tests/scenario/manager.py
@@ -295,7 +295,7 @@
             available_stores = []
             try:
                 available_stores = self.image_client.info_stores()['stores']
-            except exceptions.NotFound:
+            except lib_exc.NotFound:
                 pass
             available_import_methods = self.image_client.info_import()[
                 'import-methods']['value']