Used to quickly set modem net interfaces into their own net namespace
(named after modem USB ID path).
The idea is that since osmo-gsm-tester ofono modem.py knowns the USB
path from a modem (path yml attr), it can infer the netns from it and
run a ping process inside it.
Related: OS#2308
Change-Id: Iadb2df2974e132044fba1f1bc2db8b559912e4e1
