Support using dadiapi datasources without auth

At present, the only way to use a `dadiapi` datasource is if you either provide it with auth in the datasource config, or have an api configured in the main config. In the case where you only want to consume from a read-only dadiapi, without authenticating, this isn't possible.

See: https://github.com/dadi/web/blob/develop/dadi/lib/providers/dadiapi.js#L388-L389