Add Tempest gate job for stable/train

Now we have stable/train branch ready for devstack
and so does for all service projects.

This commit adds the Tempest testing for stable/train
by adding new jobs running on stable/train version of
openstack.

Change-Id: I0ea8008ad0ef18b9e45c41f9ba40a70fd6d57b6a

.zuul.yaml

@@ -404,6 +404,16 @@
         # without Swift, c-bak cannot run (in the Gate at least)
         c-bak: false
 
+- job:
+    name: tempest-full-train
+    parent: tempest-full
+    override-checkout: stable/train
+
+- job:
+    name: tempest-full-train-py3
+    parent: tempest-full-py3
+    override-checkout: stable/train
+
 - job:
     name: tempest-full-stein
     parent: tempest-full
@@ -622,6 +632,10 @@
         - tempest-full-py3-ipv6:
             voting: false
             irrelevant-files: *tempest-irrelevant-files
+        - tempest-full-train:
+            irrelevant-files: *tempest-irrelevant-files
+        - tempest-full-train-py3:
+            irrelevant-files: *tempest-irrelevant-files
         - tempest-full-stein:
             irrelevant-files: *tempest-irrelevant-files
         - tempest-full-stein-py3:
@@ -731,6 +745,8 @@
             irrelevant-files: *tempest-irrelevant-files
     periodic-stable:
       jobs:
+        - tempest-full-train
+        - tempest-full-train-py3
         - tempest-full-stein
         - tempest-full-stein-py3
         - tempest-full-rocky