(PUP-1596) Add config_version as an explicit env property

Legacy environments could have config_version set in their puppet.conf
stanza, just as any setting in an [env] stanza would shadow if that
setting was looked up in the context of the environment. But the new
directory environments are intended to explicitly include config_version
as a setting (albeit an optional one).

This change adds a config_version attribute to
Puppet::Node::Environment, looking it up from settings if a new call is
made, or taking it directly from a create call, which is what the Directory
environments loader will do once environment.conf is parsed for
directory environments.