[#3921] Add facts_terminus setting to Puppet settings

• defaults to "facter"
• no longer set the facts terminus class in for puppetmaster, use setting instead

Implements an abstract CouchDB terminus and a concrete CouchDB terminus
used to store node facts. Node facts are stored in a "node" document as
the "facts" attribute. This node document may also be used by other
couchdb termini that store node-related information. It is recommended
to use a separate document (or documents) to store large data structures
like catalogs, linking them to their related node document using
embedded ids.

This implementation depends on the "couchrest" gem.