(#5061) - allow special hostclass/define variables to be evaluated as defaults.

I have always been annoyed that special variables for defines and hostclasses can not be evaluated as param defaults. Special variables are: $name, $title, $module_name.

Code example:
class x (

foo = $name
) { notice($foo)}

should print x, and with my patch, it does.

Reviewed-by: Paul Berry <paul@puppetlabs.com>