Implement framerate setting for opencv

opencv finally implemented set(CV_CAP_PROP_FPS,frameRate) for v4l a while ago, so implement it in mavconn as well.
There's not a good way to programmatically check it since get(CV_CAP_PROP_FPS) is not implemented in opencv. I checked in qgs and the video became choppy and the bandwidth listed in the comm console dropped from 2mbit/s to 150kbit/s after specifying --fps 12.