Uninstall: fix BytesWarning exception
    
    When uninstalling, if server does not have all roles, exception
    is thrown as the role is not found. `member_principal` variable
    has to be string here, otherwise we're using str on bytes.
    
    https://pagure.io/freeipa/issue/4985
    
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>