(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.
Just thinking through the compatibility issues, PDB3 -> Puppet3.x will call `Facts#strip_internal`, and that will strip out the `_timestamp` fact. PDB3 -> Puppet4 will also call `Facts#strip_internal`, which doesn't generate the `_timestamp` fact anymore. PDB3 -> Puppet5 will call Facts#values (we're removing Facts#strip_internal method in PUP-3670) So looks good, :+1: