Purge default repos if specified (new default)

With regards to #27 this makes puppet-pkgng go ahead and purge /etc/pkg if authoritative is true. This seems a sensible default to be true as noted in the issue by the @xaque208.

It's fine like that as you can simply specify a repo and you're good to go anyway.
I'll have a play with that later today and make sure there's no overlap.