(Don't Merge)(FACT-753) Update default external fact directories for unified agent layout

This commit updates the default search directory for external facts.
We now look in /opt/puppetlabs/agent/facts.d for both open source and
PE versions of Facter.

This change is part of the unified agent effort.
@haus suggested the other day that maybe we want facter 2.4 to check old *and* new directory locations, to make a smoother transition for sites running puppet 3.x who update to facter 2.4. (And then in facter 3 (cfacter) dump the `/etc/`-based directories.)

Thoughts?