Add horizon-dsvm-tempest-plugin-py27 job

We now support both python 2.7 and python 3 environments,
but bug 1814028 turns out that we cannot detect python2/3 difference
only with unit tests. It sounds reasonable to me to add python 2.7
version of the tempest job. Horizon runs only one devstack job now,
so I believe it is okay to add one more job.

Change-Id: I35fb6c04f67d68109fccd29af332f291b738d9b3

.zuul.yaml

@@ -72,6 +72,13 @@
       tempest_test_regex: horizon
       tox_envlist: all
 
+- job:
+    name: horizon-dsvm-tempest-plugin-py27
+    parent: horizon-dsvm-tempest-plugin
+    vars:
+      devstack_localrc:
+        USE_PYTHON3: false
+
 - job:
     # Security testing for known issues
     name: horizon-tox-bandit-baseline
@@ -106,6 +113,7 @@
         - horizon-integration-tests:
             voting: false
         - horizon-dsvm-tempest-plugin
+        - horizon-dsvm-tempest-plugin-py27
         - horizon-tox-bandit-baseline:
             voting: false
     gate:
@@ -113,3 +121,4 @@
       jobs:
         - horizon-openstack-tox-python3-django111
         - horizon-dsvm-tempest-plugin
+        - horizon-dsvm-tempest-plugin-py27