Make the kafka group_id prefix configurable in the config file

rather than hardcoding it to 'swh-dataset-export-', use the 'group_id'
value from the 'journal' section of the config file as prefix, if given
9otherwise default to the former value).

This is needed because current auth policy of swh kafka cluster only allow
group_id to start with the actual login for authenticated connection.
So we need to be able to specify this group_id prefix.