Allow for commands that use positional parameters to add/remove attributes
    
    Commands that modify a single multivalued attribute of an entry should use
    positional parameters to specify both the primary key and the values to
    add/remove. Named options are redundant in this case.
    
    The `--certificate option` of `*-add/remove-cert` commands was turned
    mandatory to avoid EmptyModlist when it is omitted.
    
    https://fedorahosted.org/freeipa/ticket/3961
    https://fedorahosted.org/freeipa/ticket/5413
    
    Reviewed-By: David Kupka <dkupka@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>