Implement BBR holdback experiment to help diagnose unexpected ndt7 results

[mattmathis]

For some small fraction (1%?) of the ndt7 tests use CUBIC instead of BBR, to help identify the root causes of unexpected low performance.

[mattmathis]

See also #314

[laiyi-ohlsen]

Good candidate for future OKR for long term data calibration and A/B testing

[mattmathis]

I encountered another WiFi issue that might be due to an interaction with BBR. The only robust experiment to test this would be to see if CUBIC gets significantly better performance under similar scenarios.

[mattmathis]

Tentative requirements (combining #351)

• Enable the client to force CC from a list of valid names
• If no forced CC from client, Use weighted random selection.
• parameters for random selection should be set on then command line, such that they can be set in the container config.