Bug 30708: Allow modifying attribute linked with AV to another value

Bug 30708: Allow modifying attribute linked with AV to another value

If a value is not in the AV list, or if it differs slightly, they would
like to force the modification of the value, without creating a new
authorised value.

Note that this could be a candidate for an option at the attribute
level if there are different needs (ie. for some attributes we don't
want to allow an other value).

Sponsored-by: BULAC - http://www.bulac.fr/
Signed-off-by: BULAC - http://www.bulac.fr/
Signed-off-by: Heather Hernandez <heather_hernandez@nps.gov>
Signed-off-by: Laurence Rault <laurence.rault@biblibre.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>