build synchronizers by default, unless set false in xos_services dict

Change-Id: I97fa804a143ffd9e05b7d08424cce698a31c3524
diff --git a/roles/build-images/tasks/main.yml b/roles/build-images/tasks/main.yml
index c6adab6..7abaac4 100644
--- a/roles/build-images/tasks/main.yml
+++ b/roles/build-images/tasks/main.yml
@@ -56,4 +56,4 @@
     pull: False
     force: True
   with_items: "{{ xos_services }}"
-  when: not frontend_only and item.synchronizer is defined and item.synchronizer
+  when: not frontend_only and (item.synchronizer is not defined or item.synchronizer)