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: I8025ea0007c7e698789eaa65663be9ba2f5ba5b5

.zuul.yaml

@@ -5,10 +5,10 @@
     check:
       jobs:
         - ec2api-tempest-plugin-functional
+        - ec2api-tempest-plugin-functional-xena
         - ec2api-tempest-plugin-functional-wallaby
         - ec2api-tempest-plugin-functional-victoria
         - ec2api-tempest-plugin-functional-ussuri
-        - ec2api-tempest-plugin-functional-train
     gate:
       queue: ec2-api
       jobs:
@@ -41,6 +41,11 @@
       tempest_plugins:
         - ec2api-tempest-plugin
 
+- job:
+    name: ec2api-tempest-plugin-functional-xena
+    parent: ec2api-tempest-plugin-functional
+    override-checkout: stable/xena
+
 - job:
     name: ec2api-tempest-plugin-functional-wallaby
     parent: ec2api-tempest-plugin-functional
@@ -56,11 +61,6 @@
     parent: ec2api-tempest-plugin-functional
     override-checkout: stable/ussuri
 
-- job:
-    name: ec2api-tempest-plugin-functional-train
-    parent: ec2api-tempest-plugin-functional
-    override-checkout: stable/train
-
 - job:
     name: ec2api-tempest-plugin-functional-full
     parent: ec2api-tempest-plugin-functional