Loop through the datastore's datasources to find
    the datasource identified by datasource_id, return
    the matching datasource's columns.

def discover_datasource_columns(datastore_str, datasource_id):
    """ Loop through the datastore's datasources to find
    the datasource identified by datasource_id, return
    the matching datasource's columns. """
    datastore = DataStore(datastore_str)
    datasource = datastore.get_datasource(datasource_id)
    if datasource.type != "RASTER":
        return datasource.list_columns()
    else:
        return []