Bug 18298: minPasswordLength should not be used as the default password length

Bug 18298: minPasswordLength should not be used as the default password length

The length of the passwords generated for a patron should not be as long
as the value of minPasswordLength.
It is the minimum required size of a password, not the maximum!
So let's fix it to 8 if the minPasswordLength if < 8, that sounds
reasonable and less risky for patrons.

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>