parameters: remove unused ConversionError and ValidationError arguments
    
    Do not set the `value`, `index` and `rule` arguments when raising
    ConversionError and ValidationError. The arguments are unused and are not
    specified consistently accross the framework and plugins.
    
    https://fedorahosted.org/freeipa/ticket/4739
    
    Reviewed-By: David Kupka <dkupka@redhat.com>