Make changes to OOBE update tests for moblab.

The test has some hardcoded values which fail when run on moblab. I've
added a devserver_dir entry to moblab_config overriding what is present
in global_config).

BUG=chromium:936268
TEST=Keith can you try this out on your local moblab.

Change-Id: Ia369650647ffaf44e30e45d3757ac649a288a285
Reviewed-on: https://chromium-review.googlesource.com/1743047
Tested-by: David Haddock <dhaddock@chromium.org>
Commit-Ready: David Haddock <dhaddock@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Keith Haddow <haddowk@chromium.org>
diff --git a/moblab_config.ini b/moblab_config.ini
index 765a716..dcbeb9f 100644
--- a/moblab_config.ini
+++ b/moblab_config.ini
@@ -36,6 +36,7 @@
 # In moblab, this paramater is empty and uses the image bucket by default.
 results_storage_server:
 stable_cros_version: R75-12105.100.0
+devserver_dir = /usr/lib/devserver/
 
 # The pubsub topic the gs_offloader notification is sent to.
 cloud_notification_topic: projects/chromeos-partner-moblab/topics/moblab-notification