Fix Debian configuration

Hi Al,

Debian uses **TWO** configuration dirs for nginx (`/etc/nginx/sites-available` **AND** `/etc/nginx/conf.d`), and only the former was being included in the main config. 

This raised an issue when setting upstreams with the `manifests::resource::upstream` define, as it writes them to `/etc/nginx/conf.d`. As it was not being included when runnning on a Debian host, nginx failed to start.

As a side efect, all hardcoded paths in the defines can now be handled better (not the best way for my taste, but the best I could find without making extensive changes in the module)

This patches also include a spec test to prevent breaking the main `nginx.conf` file with typos.