Add id to CommandData args

This allows jobtype implementors to later identify the process in
process_stopped() or process_stdout_line() for example if multiple
processes are running.
I'd do this instead,

```python
try:
    process_protocol.id = command.id
except AttributeError:
    process_prococol.id = None
```

Seems more direct to me and will ensure `id` is set to None if `command.id` does not exist.  If that's not what we want though then we could just pass too.