Add stable/xena jobs on master gate

We have stable/xena released so we should add
their job on master gate to keep branchless tempest
plugins compatible to stable branch.

This also removes the stable/train job as that is in EM
state now.

Ref: Tempest plugins guide for stable branch testing:
- https://docs.openstack.org/tempest/latest/stable_branch_testing_policy.html

Change-Id: I41705ea681578cfdc510052122abec4838d8c578

.zuul.yaml

@@ -6,18 +6,23 @@
       jobs:
         - murano-tempest-api:
             voting: false
+        - murano-tempest-api-xena:
+            voting: false
         - murano-tempest-api-wallaby:
             voting: false
         - murano-tempest-api-victoria:
             voting: false
         - murano-tempest-api-ussuri:
             voting: false
-        - murano-tempest-api-train:
-            voting: false
         - murano-tempest-cfapi
     gate:
       queue: murano
 
+- job:
+    name: murano-tempest-api-xena
+    parent: murano-tempest-api
+    override-checkout: stable/xena
+
 - job:
     name: murano-tempest-api-wallaby
     parent: murano-tempest-api
@@ -33,9 +38,3 @@
     parent: murano-tempest-api
     nodeset: openstack-single-node-bionic
     override-checkout: stable/ussuri
-
-- job:
-    name: murano-tempest-api-train
-    parent: murano-tempest-api
-    nodeset: openstack-single-node-bionic
-    override-checkout: stable/train