Fix a filename and pillar key issue

* Ensure `common.sls` references correct template filename
* Use `pillar.get(...)` for message_do_not_modify with default

nginx/common.sls

     - user: root
     - group: root
     - mode: 440
-    - source: salt://nginx/templates/upstart_logger.jinja
+    - source: salt://nginx/templates/upstart-logger.jinja
     - context:
       type: {{ log_type }}
   service:

templates/upstart-logger.jinja

-# {{ pillar['message_do_not_modify'] }}
+# {{ pillar.get('message_do_not_modify', '') }}
 # startup script for Nginx loggers
 
 start on starting nginx