Switch to override-checkout for stable branch jobs

Change-Id: Ia4faf20e4e31787463fdcbe838e71d36f83b5f05
Depends-On: Iec7be918f75e3400197748385b287fb945d761ac
diff --git a/zuul.d/stable-jobs.yaml b/zuul.d/stable-jobs.yaml
index 5e055f9..ed9d8df 100644
--- a/zuul.d/stable-jobs.yaml
+++ b/zuul.d/stable-jobs.yaml
@@ -2,35 +2,29 @@
 - job:
     name: ironic-dsvm-standalone-pike
     parent: ironic-dsvm-standalone
-    vars:
-      branch_override: stable/pike
+    override-checkout: stable/pike
 
 - job:
     name: ironic-tempest-dsvm-ipa-wholedisk-bios-agent_ipmitool-tinyipa-pike
     parent: ironic-tempest-dsvm-ipa-wholedisk-bios-agent_ipmitool-tinyipa
-    vars:
-      branch_override: stable/pike
+    override-checkout: stable/pike
 
 - job:
     name: ironic-tempest-dsvm-ipa-wholedisk-bios-agent_ipmitool-tinyipa-ocata
     parent: ironic-tempest-dsvm-ipa-wholedisk-bios-agent_ipmitool-tinyipa
-    vars:
-      branch_override: stable/ocata
+    override-checkout: stable/ocata
 
 - job:
     name: ironic-tempest-dsvm-ironic-inspector-pike
     parent: ironic-tempest-dsvm-ironic-inspector
-    vars:
-      branch_override: stable/pike
+    override-checkout: stable/pike
 
 - job:
     name: ironic-tempest-dsvm-ironic-inspector-ocata
     parent: ironic-tempest-dsvm-ironic-inspector
-    vars:
-      branch_override: stable/ocata
+    override-checkout: stable/ocata
 
 - job:
     name: ironic-inspector-grenade-dsvm-pike
     parent: ironic-inspector-grenade-dsvm
-    vars:
-      branch_override: stable/pike
+    override-checkout: stable/pike