task/: add args.py

The usage doc string for a task is tedious to write and
hard to keep reconciled with the code as defaults are changed.
args.py includes a helper to put it all in one place.

Signed-off-by: Samuel Just <sam.just@inktank.com>
task/peering_speed_test.py: add test which summarizes pg peering speed

Running this regularly may warn us about slow peering.

Signed-off-by: Samuel Just <sam.just@inktank.com>