Fix octavia-v2-dsvm-scenario-ubuntu-xenial

Recent changes to the octavia-tempest-plugin jobs broke the
octavia-v2-dsvm-scenario-ubuntu-xenial job as it tries to use
python3.6 on a xenial nodeset job.
This patch explicitly forces python2 for this job.

Change-Id: I95fa5f2f2fd17140a0ec892265d84542d613c04a
diff --git a/zuul.d/jobs.yaml b/zuul.d/jobs.yaml
index 0d68088..8462374 100644
--- a/zuul.d/jobs.yaml
+++ b/zuul.d/jobs.yaml
@@ -559,6 +559,7 @@
       devstack_localrc:
         OCTAVIA_AMP_BASE_OS: ubuntu
         OCTAVIA_AMP_DISTRIBUTION_RELEASE_ID: xenial
+        USE_PYTHON3: false
 
 - job:
     name: octavia-v2-dsvm-tls-barbican