Add non-voting train TripleO jobs

Because we use modules starting from train, let's check we don't
break anything in train branch for TripleO.
Use non-voting more for informational purpose, since old branches
jobs can be unstable.

Change-Id: I0026b3bdd785a32b7d701ed6ac146853c0cdd162

.zuul.yaml

@@ -339,6 +339,14 @@
       - ^plugins/modules/stack.*$
       - ^plugins/module_utils/openstack.*$
 
+- job:
+    name: tripleo-ci-centos-8-standalone-train-osa
+    parent: tripleo-ci-centos-8-standalone-osa
+    voting: false
+    override-checkout: stable/train
+    vars:
+      branch_override: stable/train
+
 - project:
     check:
       jobs:
@@ -390,6 +398,9 @@
             voting: false
             dependencies: *deps_unit_lint
 
+        - tripleo-ci-centos-8-standalone-train-osa:
+            dependencies: *deps_unit_lint
+
     gate:
       jobs:
         - tox-pep8