Refactor the role/attribute member reporting code
    
    The `config` object now hosts a generic method for updating the config
    entry for desired server role configuration (if not empty). The
    duplicated code in dns/trust/vaultconfig commands was replaced by a call
    to a common method.
    
    https://pagure.io/freeipa/issue/6937
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>