(#23422) Use JSON in terminus instead of PSON

The PuppetDB API specifies that it is JSON, so we should parse it as
that and not as PSON.

Some Puppet classes (Puppet::Node and Puppet::Node::Facts) don't support
JSON serialization, so continue to use PSON serialization for them.
In Puppet 3.4.0+ they have methods to do seralization in other formats
than PSON though, so once support for older versions of Puppet is
dropped they can be seralized in JSON as well.