[autotest] Add a utility to query devserver requests and load.

Add a script to check the history of stage calls made to devserver.
Following are some sample use cases:

1. Find all stage request for autotest and image nyan_big-release/R38-6055.0.0
   in the last 10 days across all devservers.
./site_utils/devserver_history.py --image_filters nyan_big 39 6260 -l 240 \
                       --artifact_filters autotest -v
output:
================================================================================
172.19.140.2
================================================================================
Number of calls:         140
Number of unique images: 1
2014-09-12 14:06:29 nyan_big-release/R39-6260.0.0    autotest, test_suites
2014-09-12 14:06:44 nyan_big-release/R39-6260.0.0    autotest, test_suites

2. Find all duplicated stage request for the last 10 days.
./site_utils/devserver_history.py -d -l 240
output:
Detecting artifacts staged in multiple devservers.
==============================================================================
nyan_big-release/R38-6055.0.0
==============================================================================
170.21.64.22: 23  requests 2014-09-04 22:44:28 -- 2014-09-05 00:03:23
170.21.64.23: 6   requests 2014-09-04 22:48:58 -- 2014-09-04 22:49:42

Count of images with duplicated stages on each devserver:
170.21.64.22   : 22
170.21.64.23   : 11

BUG=chromium:404475
TEST=local setup

Change-Id: Ic8eb35c4239ca08663a28ad5410596b2dcdbac62
Reviewed-on: https://chromium-review.googlesource.com/216282
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
3 files changed