Add stable branches testing on keystone-tempest-plugin master gate

This patch add the jobs for stable/stein, stable/train and
stable/ussuri in keystone tempest plugin. Supported stable branch
use keystone-tempest-plugin master version to test them. Adding stable
job on master ensures that keystone-tempest-plugin master version is
compatible with stable branches testing.

Change-Id: I72bf38247f693a2efcdad2e64a8948023350ff53
diff --git a/.zuul.yaml b/.zuul.yaml
index 0356a22..b827bce 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -8,7 +8,25 @@
         - keystone-dsvm-py3-functional-federation-opensuse15:
             voting: false
         - keystone-dsvm-py3-functional-federation-opensuse15-k2k
+        - keystone-dsvm-py3-functional-ussuri
+        - keystone-dsvm-py3-functional-train
+        - keystone-dsvm-py3-functional-stein
     gate:
       jobs:
         - keystone-dsvm-py3-functional
         - keystone-dsvm-py3-functional-federation-opensuse15-k2k
+
+- job:
+    name: keystone-dsvm-py3-functional-ussuri
+    parent: keystone-dsvm-py3-functional
+    override-checkout: stable/ussuri
+
+- job:
+    name: keystone-dsvm-py3-functional-train
+    parent: keystone-dsvm-py3-functional
+    override-checkout: stable/train
+
+- job:
+    name: keystone-dsvm-py3-functional-stein
+    parent: keystone-dsvm-py3-functional
+    override-checkout: stable/stein