update transcoding API
The API returns all current config variables with the possible values
To set values for a sepcific encoder, the encoder has to be set (for example: encoder=1).
If no encoder is specified, the values will be set for encoder 0
Example for Vu+ Duo2 STB:
http://vuduo2/transcoding?bitrate=5000000&automode=Off&framerate=50000&resolution=720p&encoder=1

If there is an argument or a wrong argument value which is not supportet by the STB,
a failure will be returned.