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: I18f469a60f70afeffa470dbf9db56b64871607f1

.zuul.yaml

@@ -10,14 +10,19 @@
             voting: true
         - cloudkitty-tempest-full-v2-storage-elasticsearch:
             voting: true
+        - cloudkitty-tempest-full-v2-storage-influxdb-xena
         - cloudkitty-tempest-full-v2-storage-influxdb-wallaby
         - cloudkitty-tempest-full-v2-storage-influxdb-victoria
         - cloudkitty-tempest-full-v2-storage-influxdb-ussuri
-        - cloudkitty-tempest-full-v2-storage-influxdb-train
     gate:
       queue: cloudkitty
       jobs: *cloudkitty_tempest_jobs
 
+- job:
+    name: cloudkitty-tempest-full-v2-storage-influxdb-xena
+    parent: cloudkitty-tempest-full-v2-storage-influxdb
+    override-checkout: stable/xena
+
 - job:
     name: cloudkitty-tempest-full-v2-storage-influxdb-wallaby
     parent: cloudkitty-tempest-full-v2-storage-influxdb
@@ -32,11 +37,3 @@
     name: cloudkitty-tempest-full-v2-storage-influxdb-ussuri
     parent: cloudkitty-tempest-full-v2-storage-influxdb
     override-checkout: stable/ussuri
-
-- job:
-    name: cloudkitty-tempest-full-v2-storage-influxdb-train
-    parent: cloudkitty-tempest-full-v2-storage-influxdb
-    override-checkout: stable/train
-    vars:
-      devstack_localrc:
-        USE_PYTHON3: True