Autotest: Add logic for devserver to trigger auto-update.

This CL let devserver.py trigger CrOS auto-update for a CrOS host. The code
path now is:
afe -->
    cros_host.machine_install_by_devserver() -->
        devserver.auto_update() -->
            cros_au in Devserver API.

Involving host class is because that there're many host-specific feature
needs to be read. The whole functions are combined with CL:346199, CL:348381.

BUG=chromium:613769
TEST=locally ran 'ds.auto_update([dut], [image_path])';
Ran 'repair' and 'provision test' for DUT from local autotest instance;
Ran unittest.

 Changes to be committed:
	modified:   client/common_lib/cros/dev_server.py
	modified:   client/common_lib/cros/dev_server_unittest.py
	modified:   global_config.ini
	modified:   server/afe_utils.py
	modified:   server/hosts/cros_host.py

Change-Id: I91ce8d283d97f4bb110d7478714302c4584b171e
Reviewed-on: https://chromium-review.googlesource.com/346484
Commit-Ready: Xixuan Wu <xixuan@chromium.org>
Tested-by: Xixuan Wu <xixuan@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
diff --git a/global_config.ini b/global_config.ini
index fe36c12..9120aa6 100644
--- a/global_config.ini
+++ b/global_config.ini
@@ -392,6 +392,9 @@
 # Flags to enable/disable get control file contents in batch.
 enable_getting_controls_in_batch: False
 
+# Flags to enable/disable making devserver trigger auto-update for cros host.
+enable_devserver_trigger_auto_update: False
+
 # File for hwid key.
 HWID_KEY: no_hwid_labels