Ticket #48026 - Support for uniqueness plugin to enforce uniqueness on a set of attributes.
    
    This allows defining multiple value to uniqueness-attribute-name, that will be
    taken as a set of values to maintain uniqueness for. IE mail, mailAlternate.
    If a value exists in mail, it may not exist in any other objects mail OR
    mailAlternate.
    
    https://fedorahosted.org/389/ticket/48026
    
    Reviewed by nhosoi@redhat.com.