[gs_offloader] Add gs_offloading_enabled to global_config.

Added a new config variable: gs_offloading_enabled.
This variable sets the default setting for the --delete_only flag.

For MobLab this will allow partners to enable/disable offloading
from the global_config.

BUG=chromium:420232
TEST=unit tests & locally ran with gs_offloading_enabled set
     True/False.

Change-Id: I02077cd21da8a789039961200d4e42b44c6d6750
Reviewed-on: https://chromium-review.googlesource.com/221468
Tested-by: Simran Basi <sbasi@chromium.org>
Reviewed-by: Richard Barnette <jrbarnette@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
3 files changed