(PUP-2557) Error on node inheritance

The future parser needs to be as close as possible to what will be the 
default in Puppet 4. By making the node inheritance a warning instead of an
error, it makes it so that Puppet 4 will be significantly different from
puppet 3 with --parser future. This changes it so that both Puppet 4 and
Puppet 3 with parser future will disallow node inheritance. The error message
is retained, which should provide good guidance whenever a user does
encounter this.

This change leaves all of the tests for node inheritance only working on the
--parser current. In addition an acceptance test that simply checked variable
scoping with an ENC has been converted to an integration test along with all
of the other tests about variable scope.