(PUP-406) Deprecate stringify_ facts option

The `stringify_facts` option is going to be removed in Puppet 4.0 and we
need to let users know about deprecated features.

This commit adds in a deprecation message in the stringify_facts
description.
Merge pull request #3000 from demophoon/deprecation/master/deprecate-stringify-fact-default

(PUP-406) Deprecate stringify_ facts option