Show value before override in inherited puppetclasses parameters

This provides a way to see what will the value be if the user deletes
the override.

NOTA BENA: The user can create a new override, but it somehow fails when
it comes to updating or deleting an existing overriden lookup_value.
The SQL query generated is wrong:
Mysql::Error: Operand should contain 1 column(s):
SELECT `lookup_values`.* FROM `lookup_values`
WHERE `lookup_values`.`id` IN (5)
AND (SELECT lookup_values.* FROM lookup_values WHERE
(lookup_values.match = 'fqdn=testvm1-rvrignaud'))
ORDER BY LOWER
This may be due to the :finder_sql, but Rails fails too with
3 levels of nested associations or more
(host -> host_classes -> lookup_keys -> lookup_values).