random ports feature

Ok, I agree I've silently skipped description of ports behaviour in db fixtures (not process ones).

The fact is... if you need to pass 'mongo_proc2' as a first argument - you're bounding your 'db' fixture to the 'proc' fixture. Explicite port passing is redundant, as we can take it from proc fixture underneath - and we're doing it. To be honest, I would completely remove 'port' argument from db fixtures since they always require 'proc' fixture. I can't find any practical example of using custom port in 'db' fixture that is different from 'proc' fixture. 

So my suggestion is to make a 'port=' argument deprecated in 'db' fixtures.