Allow climate device hvac_modes default override

[maforshaw]

If the integration cannot determine the hvac_modes of a climate device from the API, it defaults to: off, heat_cool

        if modes is None:
            if HVAC_MODE_HEAT_COOL not in self.operations:
                self.operations.append(HVAC_MODE_HEAT_COOL)
            return

For my device, a heater, that's actually wrong and it should be HVAC_MODE_HEAT. It would therefore be good if the default value could be set via the config rather than hard-coded to HVAC_MODE_HEAT_COOL.