Make FFmpegWorker._get_audio_layout_mapping a private method
Use _get_audio_layout_mapping in mux_gxf method in ffmpeg worker
Support minimum audio stream requirements in ffmpeg worker

The new min_stream_lists attribute let the user specifies a list of
minimum audio stream that the output audio layout should honor. If the
input does not have enough input audio streams, empty audio streams are
created.

For example considering that audio_min_streams = (4, 8):
  - if output has 2 audio streams, 2 additional empty streams are
    created.
  - if output has 4 audio streams, no additional streams are
    created.
  - if output has 6 audio streams, 2 additional empty streams are
    created.
Add audio_min_streams option to transcode action
Add audio_min_streams option to toolbox2-transcode
Use _get_audio_layout_mapping in mux_mov method in ffmpeg worker
Do not force audio layout in mux_(flv|mp4|mpegps|mxf) methods in ffmpeg worker
Add audio_channels_per_stream option to transcode action
Add audio_channels_per_stream option to toolbox2-transcode