gstlal_reference_psd fails with --data-source=white

To reproduce:

/bin/gstlal_reference_psd --channel-name=H1=foo --data-source=white --write-psd=out.psd --gps-start-time=1185493488 --gps-end-time=1185493788

I have tracked this down to the fake function in gstlal/pipeparts/source.py. The return statement is:

  return transform.tag_inject(pipeline, caps, "instrument=%s,channel-name=%s,units=strain".format(instrument, channel_name))

It looks like a mixture of Python's string formatting methods.