Cron support, example for exporting pkgng repo and some cleaning

To get the cron support working, I had to patch portsnap for it won't allow creating the portstree from non-interactively (effectively, all it does is prevent portsnap from exiting once it detects non-interactive use).
In my own setup, I automatically export every poudriere::env as a pkgng::repo, but I didn't want to create a dependency for puppet-pkgng so I merely added an example of how to use it in the README.md.
I also added support for hardcoded make.conf's, package-lists and option-files, in case you don't want to define all this from hiera/puppet.