(#22636) Allow list of external fact directories to be appended to

Currently, the only way to override the external facts directories is to write your own Facter::Util::FactLoader and replace the existing loader with this.  This commit allows you to simply set or append to the already existing ext_facts_dirs variable which makes it much more extensible for use by other programs like puppet.
Merge pull request #536 from jjulien/feature22636

(#22636) Allow list of external fact directories to be appended to