shill: Remove trivial use of iterators

Move trivial iterations through containers from using an iterator
to using range-based for loops.  In instances where it makes sense
also use "auto" for iterators in non-trivial for loops as well.

BUG=None
TEST=Unit tests

Change-Id: I840d90fb62dc96d45f63144462b9a53b28c25ee9
Reviewed-on: https://chromium-review.googlesource.com/198051
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>