transcoder: Enable B-frames in Nvidia encoder

[jailuthra]

What does this pull request do? Explain your changes. (required)

Pull in changes from livepeer/lpms#243 around enabling B-frames for Nvidia encoder

Specific updates (required)

see commits

How did you test each of these updates (required)

Local testing with pushing segments in with the following transcoding options, and verifying output renditions have the expected H264 profiles/B-frames.

[
{
  "name": "240p0",
  "fps": 0,
  "profile": "H264High",
  "bitrate": 250000,
  "width": 426,
  "height": 240
},
{
  "name": "360p0",
  "fps": 0,
  "profile": "H264ConstrainedHigh",
  "bitrate": 800000,
  "width": 640,
  "height": 360
},
{
  "name": "480p0",
  "fps": 0,
  "profile": "H264Main",
  "bitrate": 1600000,
  "width": 854,
  "height": 480
},
{
  "name": "720p0",
  "fps": 0,
  "profile": "H264Baseline",
  "bitrate": 3000000,
  "width": 1280,
  "height": 720
}
]

Does this pull request close any open issues?

Fixes #1941

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[yondonfu]

LGTM after addressing the CHANGELOG_PENDING comment.