[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.
You have to add a value for source also here:
https://github.com/superdesk/superdesk-server/blob/master/superdesk/io/tests.py#L16