def _get_core_fields(self):
        return ['launched_at', 'instance_flavor_id', 'tenant_id',
                'os_architecture', 'os_version', 'os_distro']
Should return roles when the given argument is 'roles'
dataset/ETHPy150Open openstack/stacktach-winchester/winchester/pipeline_handler.py/UsageHandler._get_core_fields--crossover with--dataset/ETHPy150Open edelight/kitchen/kitchen/backends/tests.py/TestData.test_load_data_roles