[autotest] Allow base container to be set in global config

This allows the base container to be updated using shadow config.
Also move the base container folder to gs://abci-ssp bucket, as
chromeos-image-archive bucket deletes files older than 6 months.

BUG=chromium:471312
TEST=local run ssp test, unittest, ssp functional test:
sudo python site_utils/lxc_functional_test.py

Change-Id: Ibc73e439af92f12d774a0b87bdb28d5c3a8f493c
Reviewed-on: https://chromium-review.googlesource.com/354550
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Aviv Keshet <akeshet@chromium.org>
Commit-Queue: Dan Shi <dshi@google.com>
diff --git a/global_config.ini b/global_config.ini
index bce26ca..5eaf92d 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -72,8 +72,19 @@
 
 # Directory stores LXC containers
 container_path: /usr/local/autotest/containers
-# Download link for base image of container
+
+# `container_base` is replaced by `container_base_folder_url` and `container_base_name`
+# The setting is kept for backwards compatibility reason.
+# Download link for base image of container.
 container_base: http://storage.googleapis.com/chromeos-image-archive/autotest-containers/base.tar.xz
+
+# URL to tbe folder contains base container image.
+container_base_folder_url: http://storage.googleapis.com/abci-ssp/autotest-containers
+
+# Name of the base container.
+# `base` is the container with the smallest size, still used by MobLab.
+container_base_name: base
+
 # Enable server-side package using container
 # TODO(dshi): Change this to True after SSP via container is rolled out.
 enable_ssp_container: False