Specify bitrate or something like profile in VPx encoders used in Boombox

[FelonEkonom]

No description provided.

[varsill]

It turns out that the option with the biggest impact on the VP9 encoding speed is the -deadline realtime option. It heavily elevates the bitrate (or decreases quality if bitrate is forced).
It turns out that the vpx encoder doesn't use more than 4 CPU threads (even when thread options is passed).
Other option that can be used to improve perfromance is -speed but it doesn't affect the performance that much.
With -deadline realtime option and -speed 16 I was able to achieve x8 realtime encoding speed with bitrate around ~1500kb/s.

On contrary, the H264 encoder with -preset fast options was able to achieve x25 realtime encoding speed with ~500kb/s bitrate, but it was using ~10 CPU cores.

[FelonEkonom]

Moreover, H264 encoder with -preset ultrafast was able to reach x55 reatlime encoding speed, but with worse bitrate

[varsill]

We decided to:

1. Allow users to specify the following options in the VPX encoders: g_threads, used_cpu and set them by default to default values provided by libvpx
2. Update encoders used in membrane_transcoder_plugin so that they specify the before-mentioned options to realtime use cases
3. Use updated membrane_transcoder_plugin in Boombox