onbrisca breaks as `NUM_THREADS` is not defined

While debugging why vanilla bridges are not properly measured we found, I think, the real cause:

Mar 13 10:40:34 polyanthum python[756567]: AttributeError: 'Config' object has no attribute 'NUM_THREADS'

It seems our onbasca/onbrisca mixing is biting us here.

Adding NUM_THREADS = 3 to onbrisca's default.py solves that issue.