ExecutePythonHack should return a non-primitive struct

[mbushkov]

At the moment ExecutePythonHack returns a plain string as a result. This is likely the only GRR flow that returns a primitive value instead of a proper protobuf-based struct.

This behavior confuses GRR API client, since it doesn't expect primitive types to be returned as flow results. It can't deserialize it and puts UnknownProtobuf into the payload.

[Łukasz Hanuszczak]

This has been fixed and is available since 02d3b45.