Autotest: Make autotest call the new devserver API list_suite_controls

Related to CL:338931, this CL is to change the code in autotest to call
the API: list_suite_controls, to get the control contents of all control
files in one call to devserver.

BUG=chromium:602562
TEST=Ran dev_server_unittest, control_file_getter_unittest,
suite_unittest.py, site_rpc_interface_unittest for
ENABLE_CONTROLS_IN_BATCH=False/True;
Ran test_suite:dummy in local autotest instance using local devserver
for ENABLE_CONTROLS_IN_BATCH=False/True.

Change-Id: I075fad07b79827a5d78b7de371bba845fcdf09b1
Reviewed-on: https://chromium-review.googlesource.com/339480
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Xixuan Wu <xixuan@chromium.org>
Reviewed-by: Dan Shi <dshi@google.com>
diff --git a/global_config.ini b/global_config.ini
index afc3970..c5593dd 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -366,6 +366,9 @@
 # Flags to enable/disable SSH tunnel connection for moblab host.
 enable_ssh_tunnel_for_moblab: False
 
+# Flags to enable/disable get control file contents in batch.
+enable_getting_controls_in_batch: False
+
 # File for hwid key.
 HWID_KEY: no_hwid_labels