Autotest: Add option to disable drone calling build_externals.

Added a new global_config option to disable drone_utility from calling
build_externals on initialize.

BUG=chromium:345011
TEST=Checked locally that by default build_externals is called unless
set in the shadow_config to False.

Change-Id: I35048c79d2824d546fe3da1b92245a00330192b7
Reviewed-on: https://chromium-review.googlesource.com/188779
Reviewed-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
diff --git a/global_config.ini b/global_config.ini
index f2b41f0..6abc1b4 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -111,6 +111,7 @@
 # repairing -> requeued -> fail -> repairing
 max_repair_limit: 2
 max_provision_retries: 1
+drone_build_externals: True
 
 
 [HOSTS]