Add definition check of disable_service_firewall (kubernetes-sigs#7995)

When not specifying disable_service_firewall, the task is failed.
This adds the definition check.

Author: oomichi

contrib/os-services/roles/prepare/tasks/main.yml

@@ -20,4 +20,4 @@
       "'ufw.service' in services"
 
   when:
-  - disable_service_firewall
+  - disable_service_firewall is defined and disable_service_firewall