Allow a custom nginx.conf source to be defined

A legacy configuration I was working with recently was very complex. Using vhost and location definitions left me with something more complex and harder to maintain than the original file itself (and twice as long). In some cases, some of the techniques weren't supported at all, or I'd need to implement it differently to get around validations, which was making it all the more complex. I decided to simply move the existing vhost configuration files into place using a `file` resource and notify `nginx::service`, as this was a much cleaner solution. 

I find myself in the same situation for the main `nginx.conf`, but I don't want to drop use of the module completely, so I created this patch. 

This may be something you don't want to even merge in or you may want to do it differently. It may be considered out of scope of the module to only use it only for the package and service management. FWIW, I use all parts of this module in other scenarios and it works great, thanks in advance!