Fix tempest-slow-py3 job for < stable/xena

tempest-slow-py3 job started using the new tox env
'slow' which is not present in old tempest used in
< stable/xena. This commit preapre a separate job
definiton for stable testing which use the old tox
env.

Closes-Bug: #2027817
Change-Id: Ibe603004ef1a8694a9c6b354b85189273787b3f3
diff --git a/zuul.d/stable-jobs.yaml b/zuul.d/stable-jobs.yaml
index 89435ce..d399556 100644
--- a/zuul.d/stable-jobs.yaml
+++ b/zuul.d/stable-jobs.yaml
@@ -311,6 +311,18 @@
           USE_PYTHON3: true
 
 - job:
+    name: tempest-slow-py3
+    parent: tempest-slow
+    # This job version is to use the 'slow-serial' tox env for
+    # the stable/ussuri to stable/wallaby testing.
+    branches:
+      - stable/ussuri
+      - stable/victoria
+      - stable/wallaby
+    vars:
+      tox_envlist: slow-serial
+
+- job:
     name: tempest-full-py3-opensuse15
     parent: tempest-full-py3
     nodeset: devstack-single-node-opensuse-15