(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.