(PUP-1735) Reimplement deprecated Puppet::Node::Environment.current

Puppet::Node::Environment.current is used in some external code, so
reimplement it, linking to the context lookup for the current environment
and issue a deprecation warning.
Merge pull request #2382 from domcleal/tickets/master/pup-1735-current-env

(PUP-1735) Reimplement deprecated Puppet::Node::Environment.current