cast_volume: pylint errors

pylint gives the following errors:

************* Module nabu.app.cast_volume
nabu/app/cast_volume.py:147:36: E1123: Unexpected keyword argument 'output_format' in constructor call (unexpected-keyword-arg)
nabu/app/cast_volume.py:152:36: E1123: Unexpected keyword argument 'output_format' in constructor call (unexpected-keyword-arg)
nabu/app/cast_volume.py:157:36: E1123: Unexpected keyword argument 'output_format' in constructor call (unexpected-keyword-arg)

related: !223 (merged)