Update to native GPU support in Docker

[alfpark]

Update to latest Docker release with "native" GPU support.

• [x] Update Docker CE to 19.03.1
• [x] Update nodeprep with nvidia-container-toolkit and drop nvidia-docker2
• [x] Non-native Docker GPU execution (flag switch)
• [x] jobs.yaml breaking change
  • [x] Change gpu to gpus
  • [x] Accept text, either number or arbitrary string
  • [x] Unset, empty/null will default to all if on a GPU pool
  • [x] disable will disable invoking the parameter (same as old gpu: false behavior)
  • [x] Otherwise take value as-is with parameter
• [x] Ensure native compatibility
• [x] Update docs
  • [x] Custom image docs
  • [x] jobs docs and config template
  • [x] Scan recipes and fixup