[autotest] Add -m option to gs_offloader

gs_offloader used -m option by default when running gsutil.
Make -m an option of gs_offloader, when it is enabled, then gs_offloader
uses -m option for gsutil. Otherwise, run gsutil without -m.

BUG=chromium:513467
TEST=Run gs_offloader in local machine and see gsutil is run
with/without -m option.
site_utils/gs_offloader_unittest.py
DEPLOY=gs_offloader,gs_offloader_s

Change-Id: If4db154f5c5e9649c032be6236eececec363e7c0
Reviewed-on: https://chromium-review.googlesource.com/292811
Trybot-Ready: Mungyung Ryu <mkryu@google.com>
Tested-by: Mungyung Ryu <mkryu@google.com>
Reviewed-by: Mungyung Ryu <mkryu@google.com>
Commit-Queue: Mungyung Ryu <mkryu@google.com>
2 files changed