docs: def update_instance(self, name, settings):
          """ Updates settings of instance <name> with the
              dictionary <settings>. """
          if name not in self.insts:
              raise ValueError("There's no instance named %s" % name)
          if 'module' in settings:
              raise ValueError(("Can't change module of existing instan"
                                + "ce %s") % name)
          self.l.info('update instance %-15s' % (name))
          for k, v in six.iteritems(settings):
              self.change_setting(name, k, v)