cashew: use nameservers from the ipconfig object

Use nameservers from the ipconfig object when fetching usage
information from a data plan provider.

BUG=chromium-os:27904
TEST=fetch usage information while attached to Ethernet

Change-Id: I2fcbaa060ae940bb76b29acabcfa0c8ff260461c
Reviewed-on: https://gerrit.chromium.org/gerrit/18803
Commit-Ready: Jason Glasgow <jglasgow@chromium.org>
Reviewed-by: Jason Glasgow <jglasgow@chromium.org>
Tested-by: Jason Glasgow <jglasgow@chromium.org>
8 files changed