to reduce duplication of constants

To resolve the occurrence of the below error, a dependency has been added to require the nginx user first be created, either by nginx package installation or explicitly with salt (when installing nginx from source).

```
----------
          ID: /var/www
    Function: file.directory
      Result: False
     Comment: User nginx is not available Group nginx is not available
     Started: 15:50:02.070799
    Duration: 0.531 ms
     Changes:
----------
```

Fixes GH saltstack-formulas/nginx-formula#14.

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

* Re-organize according to http://docs.saltstack.com/topics/conventions/formulas.html#writing-formulas