cashew: Allow specification of nameservers in http_fetcher

Allow specification of nameservers in http_fetcher

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

Change-Id: I7222c4c852b3b70dc09c52b49a1633a3529849a7
Reviewed-on: https://gerrit.chromium.org/gerrit/18800
Reviewed-by: Nathan J. Williams <njw@chromium.org>
Commit-Ready: <jglasgow@google.com>
Tested-by: <jglasgow@google.com>