Add vod_log_level error in nginx settings.

[kinbov]

Any image request on modern versions of FFmpeg results in an entry in error.log, even if error_log is set in Nginx:

 /var/log/nginx/error.log error;

I'd like to be able to customize this output in Nginx, for example:

 vod_log_level error; # or warn, info, debug

It really clutters up the error log with things that shouldn't be there:

[swscaler @ 0x345f540] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 0x26dd840] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 0x26fe380] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 0x25bbd00] deprecated pixel format used, make sure you did set range correctly
[swscaler @ 0x2aed080] deprecated pixel format used, make sure you did set range correctly