Ticket 49231 - force EXTERNAL always
Bug Description: Because of how our sasl code works, EXTERNAL bypasses
a number of checks so is always available.
Fix Description: Force EXTERNAL to the present mech list, regardless
of the whitelist.
https://pagure.io/389-ds-base/issue/49231
Author: wibrown
Review by: mreynosd (Thanks!)
(cherry picked from commit e6e0db35842fc6612134cff5a08c4968230d1b2f)