Fix autobackup-property has illegal value: 'on'

YAML works in curious ways

tasks/system_cfg/backup_dest.yml

@@ -65,7 +65,7 @@
   vars:
     server_properties:
       - key: "autobackup:bkp_{{ hostvars[item]['inventory_hostname'] }}"
-        value: true
+        value: "true"
       - key: mountpoint
         value: "/home/{{ backup_user }}/repo/{{ hostvars[item]['inventory_hostname'] }}"
       - key: sharenfs