waterfall_layout_dump works well for local test and try job test. not sure why it's not working in config-updater env. adding logs to investigate. 

not sure if it's related to GCE env. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/013a3a9d282f420f14b4a6f87cc72e4658123a19

commit 013a3a9d282f420f14b4a6f87cc72e4658123a19
Author: Ningning Xia <nxia@chromium.org>
Date: Fri Jul 22 18:55:09 2016

Remove "important" and add logging for config-updater.

1. As config-updater is alive, we can remove "important" flag in
overwritten_configs.
2. Add logging to investigate updating the waterfall_layout_dump problem
in config-updater.

BUG= chromium:630685 
TEST=cbuildbot_run;run_tests

Change-Id: Ic1a742ee6d345ddc65ebd039b821e2853a823474
Reviewed-on: https://chromium-review.googlesource.com/362670
Commit-Ready: Ningning Xia <nxia@chromium.org>
Tested-by: Ningning Xia <nxia@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>

[modify] https://crrev.com/013a3a9d282f420f14b4a6f87cc72e4658123a19/cbuildbot/waterfall_layout_dump.txt
[modify] https://crrev.com/013a3a9d282f420f14b4a6f87cc72e4658123a19/cbuildbot/stages/config_stages.py
[modify] https://crrev.com/013a3a9d282f420f14b4a6f87cc72e4658123a19/cbuildbot/chromeos_config.py

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/chromite/+/84a2adf150930d294bf793e5cb6796fff8a0730d

commit 84a2adf150930d294bf793e5cb6796fff8a0730d
Author: Ningning Xia <nxia@chromium.org>
Date: Tue Aug 02 05:03:50 2016

Fix waterfall_layout_dump in config-updater.

Clear _CACHED_CONFIG before executing cmd waterfall_layout_dump.

BUG= chromium:630685 
TEST=run_tests

Change-Id: I7e658faf8361eb70df07d7813dca15f8ab56f0b7
Reviewed-on: https://chromium-review.googlesource.com/365301
Commit-Ready: Ningning Xia <nxia@chromium.org>
Tested-by: Ningning Xia <nxia@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>

[modify] https://crrev.com/84a2adf150930d294bf793e5cb6796fff8a0730d/cbuildbot/stages/config_stages.py