Allow overriding Facter config (specifically external facts dir) when calling Puppet

Currently when calling puppet (e.g `puppet facts`) you can override the directory from which custom facts in ruby are loaded (using the `--factdir` flag or the FACTERLIB environment variable), however it does not appear to be possible to customize the location from which external facts are loaded other than by editing facter.conf.

When calling `facter` directly as a command line tool, you can of course specify the `--external-dir` flag at the command line, but there is no equivalent flag when calling `puppet facts` (or `puppet agent`, `puppet catalog`, etc), nor is there an environment variable you can set to achieve this end.