Convert utils.nodesetup to utils.WriteFile(data=…)

It makes no sense to iteratively write the new etc/hosts file, as we
can pre-compute the desired contents (neither the old nor the new
versions are safe against concurrent changes anyway).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>