This is follow-on work from  issue 678430 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/autotest/+/7fc39c548b05bc8a495230582aa5bc1d187ad9ee

commit 7fc39c548b05bc8a495230582aa5bc1d187ad9ee
Author: Prathmesh Prabhu <pprabhu@chromium.org>
Date: Sat Apr 07 05:00:02 2018

autotest: Add option to pass FileStore to autoserv

This CL adds a new argument --host-info-subdir to autoserv to provide a
subdirectory within the results directory where prepopulated HostInfo
stores reside for the targeted machines.

If provided, these stores will be used for and updated by the autoserv
execution. The default (when the option is not provided) is to
initialize stores backed by the AFE.

BUG=chromium:821087
BUG= chromium:827388 
TEST=Run task on skylab-drone; Run provision on moblab-vm

Change-Id: I7bc01f135553364a91f44617c6d1cfa319442322
Reviewed-on: https://chromium-review.googlesource.com/974361
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Allen Li <ayatane@chromium.org>

[modify] https://crrev.com/7fc39c548b05bc8a495230582aa5bc1d187ad9ee/server/server_job.py
[modify] https://crrev.com/7fc39c548b05bc8a495230582aa5bc1d187ad9ee/server/server_job_unittest.py
[modify] https://crrev.com/7fc39c548b05bc8a495230582aa5bc1d187ad9ee/server/autoserv_parser.py
[modify] https://crrev.com/7fc39c548b05bc8a495230582aa5bc1d187ad9ee/server/autoserv

This CL may need to be cherry-picked back to beta, stable branches in order for skylab to work with older tests with SSP. Will reopen if we need that.