Use Tempest compatible version for xenial node job

Tempest is py3 only now and xenial node has py3.5.
We tried to install the py3.6 on xenial node via devstack
but revert that solution. Later we did pin the Tempest
with compatible version on xenial which is 23.0.0

- https://github.com/openstack/devstack/blob/8cd4e9a9076e05974ed1242ccab3391c55c7070c/stackrc#L316

Change-Id: I78e184c6cb2b1448ef243766a20072baa869d1b3
diff --git a/zuul.d/jobs.yaml b/zuul.d/jobs.yaml
index 8462374..1e2e347 100644
--- a/zuul.d/jobs.yaml
+++ b/zuul.d/jobs.yaml
@@ -560,6 +560,7 @@
         OCTAVIA_AMP_BASE_OS: ubuntu
         OCTAVIA_AMP_DISTRIBUTION_RELEASE_ID: xenial
         USE_PYTHON3: false
+        TEMPEST_BRANCH: 23.0.0
 
 - job:
     name: octavia-v2-dsvm-tls-barbican