(PDB-971) retire facts.strip_internal

This patch adds a maybe_strip_internal method to Puppet::Node::Facts::Puppetdb
that will call Facts#strip_internal if the method exists, but Facts#values if
not. This will allow our terminus to remain backwards compatible when Puppet
retires the strip_internal method and the _timestamp fact.
Merge pull request #1154 from wkalt/ticket/master/pdb-971-retire-facts-strip-internal

(PDB-971) retire facts.strip_internal