TEMPEST_PY26_NOSE_COMPAT in py26 jobs

Use py26 skip exception compatibility code path in py26 jobs.

Change-Id: Id6c29f26f19f69abab558d5f94d8cbf6190ed43b
diff --git a/tox.ini b/tox.ini
index abc9e42..ff09b3f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -51,6 +51,7 @@
          NOSE_OPENSTACK_YELLOW=3
          NOSE_OPENSTACK_SHOW_ELAPSED=1
          NOSE_OPENSTACK_STDOUT=1
+         TEMPEST_PY26_NOSE_COMPAT=1
 commands =
   nosetests --logging-format '%(asctime)-15s %(message)s' --with-xunit -sv --xunit-file=nosetests-full.xml tempest/api tempest/scenario tempest/thirdparty tempest/cli tempest/tests {posargs}
 
@@ -62,6 +63,7 @@
          NOSE_OPENSTACK_YELLOW=3
          NOSE_OPENSTACK_SHOW_ELAPSED=1
          NOSE_OPENSTACK_STDOUT=1
+         TEMPEST_PY26_NOSE_COMPAT=1
 commands =
   nosetests --logging-format '%(asctime)-15s %(message)s' --with-xunit -sv --attr=type=smoke --xunit-file=nosetests-smoke.xml tempest {posargs}