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.
We've got a purge_repos_d option here already that in my head should do the same thing as the authoritative here.  The word authoritative is not clear to me what action will be taken when set to true or false.  Could we please piggy back on the existing variable if it solves the same use case?