process-job-source-groups.py does not pass log level or --log-file information to children

[Gavin Panella]

Passing -v, -q or --log-file options to process-job-source-groups.py
have little effect; only -v or nothing is passed through to child
processes.

The way c.l.scripts.logger works makes it difficult to figure out what
to pass through (-v and -q are connected to callbacks that change the
main logger's log level, so figuring out how many -v or -q flags to
pass to children requires some maths - see LogLevelNudger).

Bonus points: perhaps process-job-source-groups.py ought to do
something cleverer with log files, e.g. add the job source name as a
suffix to the log file path when spawning children.