Move stable branch testing to oldest supported branch

Currently we test the stable branch with stable/train
which is in EM phase and also failing. Stackviz need to
test if supported branch works or not so let's test the
oldest stable supported branch. Considering stable/yoga
is going to EM/Unmaintained soon, I am adding stable/zed
job.

Change-Id: I5bf251f438317636aa0d2598718fe08a1aa97683

.zuul.yaml

@@ -20,24 +20,6 @@
     required-projects:
       - openstack/stackviz
 
-- job:
-    name: tempest-full-rocky
-    parent: tempest-full-py3
-    nodeset: openstack-single-node-xenial
-    override-checkout: stable/rocky
-
-- job:
-    name: tempest-full-stein
-    parent: tempest-full-py3
-    nodeset: openstack-single-node-bionic
-    override-checkout: stable/stein
-
-- job:
-    name: tempest-full-train
-    parent: tempest-full-py3
-    nodeset: openstack-single-node-bionic
-    override-checkout: stable/train
-
 - project:
     templates:
       - publish-openstack-docs-pti
@@ -62,7 +44,7 @@
             - ^.coveragerc$
             - ^.gitignore$
             - ^.gitreview$
-      - tempest-full-train:
+      - tempest-full-zed:
           irrelevant-files: *irrelevant-files
     gate:
       jobs: