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

zuul.d/integrated-gate.yaml

@@ -300,7 +300,10 @@
     # This job version is with swift enabled on py3
     # as swift is ready on py3 from stable/ussuri onwards.
     timeout: 10800
-    branches: ^(?!stable/(ocata|pike|queens|rocky|stein|train)).*$
+    # As the 'slow' tox env which is not available in old tempest used
+    # till stable/wallaby, this job definition is only for stable/xena
+    # onwards and separate job definition until stable/wallaby
+    branches: ^(?!stable/(ocata|pike|queens|rocky|stein|train|ussuri|victoria|wallaby)).*$
     vars:
       tox_envlist: slow
       devstack_localrc:

zuul.d/stable-jobs.yaml

@@ -310,6 +310,18 @@
         devstack_localrc:
           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