Update grenade setting in stable/xena

Along with updating the grenade setting in
stable/xena, adding the forward upgrade job as experimental.

Change-Id: I2782577c3ffc3c0f3e53afae4081ac0c6906da5a

.zuul.yaml

@@ -17,6 +17,7 @@
         - octavia-grenade
     experimental:
       jobs:
+        - grenade-forward
         - grenade-postgresql
 
 - job:
@@ -331,7 +332,8 @@
     vars:
       # When a stable branch stable/X is created from master,
       # this value should be updated to stable/X.
-      grenade_from_branch: master
+      grenade_from_branch: stable/xena
+      grenade_to_branch: master
 
 - job:
     name: grenade-dsvm-neutron-forward

roles/configure-grenade-branches/defaults/main.yaml

@@ -1,3 +1,3 @@
 ---
 grenade_from_branch: stable/wallaby
-grenade_to_branch: master
+grenade_to_branch: stable/xena