network script used in multi_nic test dumps trace when using --device-name

according to Daniel's testing, the 'network' script performs fine when doing the actual network testing on the ARM server. However, when you do the info stuff that the script is capable of, tracebacks are seen in output. Specifically, when getting device-name from sysfs, the script fails because the info it's looking for does not exist in sysfs.

Here are results for each option that fails for 'network info -i eth0 --device-name'

ubuntu@c18:/usr/share/checkbox/scripts$ ./network info -i eth0 --device-name
Traceback (most recent call last):
  File "./network", line 382, in <module>
    main()
  File "./network", line 378, in main
    args.func(args)
  File "./network", line 319, in interface_info
    key + ":", getattr(Interface(args.interface), key),
  File "./network", line 237, in device_name
    return self._read_data("device/label")
  File "./network", line 195, in _read_data
    return open(os.path.join(self.dev_path, type)).read().strip()
FileNotFoundError: [Errno 2] No such file or directory: '/sys/class/net/eth0/device/label'