gce_au_worker: allow lakitu_next to specify custom gce_tests.json

BUG=None
TEST=trybot

Change-Id: I578115ad1c61945a0dae9e0619ca889619c942c6
Reviewed-on: https://chromium-review.googlesource.com/323816
Commit-Ready: Aditya Kali <adityakali@google.com>
Tested-by: Aditya Kali <adityakali@google.com>
Reviewed-by: Daniel Wang <wonderfly@google.com>
diff --git a/lib/constants.py b/lib/constants.py
index 14e3063..9c01596 100644
--- a/lib/constants.py
+++ b/lib/constants.py
@@ -28,5 +28,6 @@
 GCS_BUCKET = 'chromeos-test-gce-tarballs'
 
 TRUSTED_BOARDS = [
-    'lakitu'
+    'lakitu',
+    'lakitu_next',
 ]