[autotest] log the user that lock/unlock a dut to metaDB

BUG=None
TEST=local setup, lock and unlock couple times, then check metaDB to see the
data.
http://172.25.61.45:9200/_plugin/elastic-hammer/
with query:
{"query": {"bool": {"minimum_should_match": 1,
 "should": [{"term": {"_type": "lock_history"}}]}},
 "size": 100,
 "sort": [{"time_recorded": "desc"}]}
Confirm response has entries like:
changed_by: "debug_user"
hostname: "172.27.213.193"
locked: true
time_recorded: 1413327364.848333

Change-Id: I643d5aefa37494170000729e2083dad799e1b8f5
Reviewed-on: https://chromium-review.googlesource.com/223371
Commit-Queue: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Fang Deng <fdeng@chromium.org>
2 files changed