Add possibility to specify soliscloud_inverter_id and soliscloud_inverter_sn in configuration

See this discussion.

For some users getting the soliscloud_inverter_id and soliscloud_inverter_sn does not work with the UserStationList API.
Configuration items have been added to soliscloud_to_pvoutput.cfg, so you can specify both yourself, so the UserStationList and inverterList API call is skipped and the configured soliscloud_inverter_id and soliscloud_inverter_sn are immediately used.