[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.
The "type" relates to ingest providers. 
It is possible that AFP/Reuters could supply data from different sources/channel. We need a mechanism at the ingest provider level to identify source/channel. The content/item will also have this "source" and it will be set by the ingest provider.  For manually created content, I guess it might be derived from "Desk" attributes.