Make job USE_PYTHON3

Change-Id: I26f8f9fcdcf9cc18f947440d18e8bb3c63db1e1f
diff --git a/.zuul.yaml b/.zuul.yaml
index f765840..571c930 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -29,6 +29,7 @@
         tempest: true
         tls-proxy: false
       devstack_localrc:
+        USE_PYTHON3: True
         TEMPEST_PLUGINS: '"/opt/stack/telemetry-tempest-plugin /opt/stack/heat-tempest-plugin"'
         GNOCCHI_ARCHIVE_POLICY_TEMPEST: "ceilometer-high-rate"
         CEILOMETER_BACKEND: "gnocchi"