Following suggestions from [1] for live streaming
this patch changes the H264 encoding setup to use
a VBV (constrained encoding) + CRF (constant rate factor).
This fixes the high limit of the output stream bitrate
as a priority over the quality as for a live streaming
the bitrate as a high impact on packet loss.

Also increase the default maximal bitrate to 1000kbit/s,
as it's almost a upload bitrate standard today.
And the maximal bitare is increased to 3000kbit/s for thus
with a good network.

[1] http://slhck.info/video/2017/03/01/rate-control.html

Change-Id: I7b38e54b45e55e7acc48cd40c354672b6507f9fd