Add uniqueness constraint on CA ACL name
    
    It is possible to add caacl entries with same "name" (cn).  The
    command is supposed to prevent this but direct LDAP operations allow
    it and doing that will cause subsequent errors.
    
    Enable the DS uniqueness constraint plugin for the cn attribute in
    CA ACL entries.
    
    Fixes: https://pagure.io/freeipa/issue/7304
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>