Switching to oslo importutils in the stress tests

The importutils is helper module for dynamic class and module,
 loading.

The importutils is compatible with python 2.6 and it is
used in multiple other openstack projects.

The previously used importlib defined with python 2.7.

Change-Id: I4e99ccf9d51cea28584c9d28a4aceab633b3f998
diff --git a/openstack-common.conf b/openstack-common.conf
index 8568f22..ff84404 100644
--- a/openstack-common.conf
+++ b/openstack-common.conf
@@ -4,6 +4,7 @@
 module=install_venv_common
 module=lockutils
 module=log
+module=importlib
 
 # The base module to hold the copy of openstack.common
 base=tempest