Use older run-tempest for stable branches

run-tempest is changed recently to add the new variables but
keep supporting the old ones too, for example:
tempest_black_regex, tempest_exclude_regex. and if both
old and new var are used in job definition (parent and child) then
new variables are picked. Because Tempest is branchless, zuul pick
the Tempest master playbooks/roles. That is why job running on stable
branch gate will pick the base job definition from Tempest master.
This way if any stable jobs which were defining the old var and using old
Tempest are broken if any of their parent job define the new var.

This commit pin the older run-tempest role for such stable branches.

Change-Id: If49ab0c31aca5b7837636727096a9bc83f891b1b
diff --git a/playbooks/devstack-tempest-ipv6.yaml b/playbooks/devstack-tempest-ipv6.yaml
index d56fb73..568077e 100644
--- a/playbooks/devstack-tempest-ipv6.yaml
+++ b/playbooks/devstack-tempest-ipv6.yaml
@@ -16,4 +16,17 @@
     # address is IPv6 etc. This is invoked before tests are run so that we can
     # fail early if anything missing the IPv6 settings or deployments.
     - devstack-ipv6-only-deployments-verification
-    - run-tempest
+  tasks:
+    - name: Run Tempest version <= 26.0.0
+      include_role:
+        name: run-tempest-26
+      when:
+        - zuul.branch is defined
+        - zuul.branch in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky", "stable/stein"]
+
+    - name: Run Tempest
+      include_role:
+        name: run-tempest
+      when:
+        - zuul.branch is defined
+        - zuul.branch not in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky", "stable/stein"]
diff --git a/playbooks/devstack-tempest.yaml b/playbooks/devstack-tempest.yaml
index 3b969f2..269999c 100644
--- a/playbooks/devstack-tempest.yaml
+++ b/playbooks/devstack-tempest.yaml
@@ -29,9 +29,17 @@
             (run_tempest_cleanup is defined and run_tempest_cleanup | bool) or
             (run_tempest_fail_if_leaked_resources is defined and run_tempest_fail_if_leaked_resources | bool)
 
+    - name: Run Tempest version <= 26.0.0
+      include_role:
+        name: run-tempest-26
+      when: (zuul.branch is defined and zuul.branch in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky", "stable/stein"]) or
+            (zuul.override_checkout is defined and zuul.override_checkout in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky", "stable/stein"])
+
     - name: Run Tempest
       include_role:
         name: run-tempest
+      when: (zuul.branch is defined and zuul.branch not in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky", "stable/stein"] and zuul.override_checkout is not defined) or
+            (zuul.override_checkout is defined and zuul.override_checkout not in ["stable/ocata", "stable/pike", "stable/queens", "stable/rocky", "stable/stein"])
 
     - name: Run tempest cleanup dry-run
       include_role: