media/video: use crf rather than bitrate

Dynamic quality change was done by increase/decreasing encoder bitrate.
It led to video artefact issues, specialy at low bitrates.
In addition, it does not make sence to use same bitrate with different
resolutions and fps. With high resolutions and fps, maximal bitrate
need to be higher.

It is possible for H264 and VP8 to set video quality using CRF.
This parameter solves video artefact issues and choose a different
output bitrate depending on input resolution and fps.

Tuleap: #80
Change-Id: I07d37175dfc335b5fc70cf54798f82eafe921ea9