Add job definitions for Yoga branch

Create job template for Yoga jobs: neutron-tempest-plugin-jobs-yoga and
fill it with job definitions for all Neutron core and stadium projects
active during the Yoga cycle.
As extra, fix a nit in xena job definitions, change stable_xena to
stable/xena.

Change-Id: I117c93e812a5476611801893ab9b746d102a74bb
diff --git a/zuul.d/xena_jobs.yaml b/zuul.d/xena_jobs.yaml
index 5ad63f0..3d8ce43 100644
--- a/zuul.d/xena_jobs.yaml
+++ b/zuul.d/xena_jobs.yaml
@@ -108,9 +108,9 @@
 - job:
     name: neutron-tempest-plugin-scenario-openvswitch-iptables_hybrid-xena
     parent: neutron-tempest-plugin-scenario-openvswitch-iptables_hybrid
-    override-checkout: stable-xena
+    override-checkout: stable/xena
     vars:
-      branch_override: stable-xena
+      branch_override: stable/xena
       network_api_extensions: *api_extensions
       network_available_features: *available_features
       devstack_localrc: