[autotest] Set ssh loglevel to ERROR for tests running in container.

This change is to avoid logs being flooded with warning `Permanently added
'[hostname]' (RSA) to the list of known hosts.`

BUG=chromium:478364
TEST=local run to confirm log has no such warning message:
http://dshi.mtv/results/589-dshi/172.27.215.232/debug/autoserv.DEBUG

Change-Id: I63c124f96eb7f36a7dca5b9673716beb0ac24fe3
Reviewed-on: https://chromium-review.googlesource.com/266401
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
Trybot-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
1 file changed