(PDB-337) Remove extraneous _timestamp fact

Previously a _timestamp fact was submitted to puppetDB even though _timestamp
was originally intended for internal use.  This commit strips internal data
(all preceded by "_") from the factset before submission to PuppetDB.
Merge pull request #993 from wkalt/ticket/master/pdb-337-fix-_timestamp-fact

(PDB-337) Remove extraneous _timestamp fact