[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>
diff --git a/global_config.ini b/global_config.ini
index 8cf008b..e56b661 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -184,6 +184,7 @@
 stable_cros_version: R39-6298.0.0
 stable_build_pattern: %s-release/%s
 source_tree: /usr/local/google/chromeos
+gs_offloading_enabled: True
 image_storage_server: gs://chromeos-image-archive/
 results_storage_server: gs://chromeos-autotest-results/
 # dev_server_hosts is the list of all servers running a devserver instance