only rebuild xos base image when xos_container_rebuild is set

Change-Id: Iba2a5d5a743626ad392f111488e3da040e53b38f
diff --git a/cord-head-playbook.yml b/cord-head-playbook.yml
index 52cca82..7e8c221 100644
--- a/cord-head-playbook.yml
+++ b/cord-head-playbook.yml
@@ -51,7 +51,7 @@
 - name: XOS setup
   hosts: head
   roles:
-    - xos-build
+    - { role: xos-build, when: xos_container_rebuild }
     - xos-install
 
 - name: Finish OpenStack install