[SD-944] Adding source to the ingest provider.

I have not modified the `class AddProvider` in the [add_provider.py](https://github.com/superdesk/superdesk-server/blob/master/superdesk/io/commands/add_provider.py). It seems that nobody has modified after services and resources came into existence. If we are going to use UI only for ingest providers then do we this command.