Add custom user and key server

- Adding ability to define custom keyserver (so that port 80 can be used when port 11371 is not open).
- Allowing people to define the user in which the aptly repository will be create 
Would it be better to move this param to the main `aptly` class and look it up from there? Rather than needing to specify it for each `aptly::mirror` and `aptly::repo` resource that you use. I figure it's unlikely that you'd mix users on the same system.