Correct import of keystoneauth1 session

keystoneclient.session has been long deprecated in favor of
keystoneauth1.session. This change corrects the import in the
tests to use the correct library's session.

Change-Id: Ic24ebde59e4b9eb70d6f14c1e0536f8d24f0de73

Author: ghugo

openstackclient/tests/unit/api/test_compute_v2.py

@@ -13,7 +13,7 @@
 
 """Compute v2 API Library Tests"""
 
-from keystoneclient import session
+from keystoneauth1 import session
 from osc_lib import exceptions as osc_lib_exceptions
 from requests_mock.contrib import fixture