Teach the devserver to stage factory images.

Changes to the devserver that allow us to stage images
from the factory branch.

TEST=Staged factory images and reimaged a device.
BUG=chromium:252503

Change-Id: I51f710996e63b4a8d3a772688cdc5643a423b511
Reviewed-on: https://gerrit.chromium.org/gerrit/64034
Commit-Queue: Prashanth Balasubramanian <beeps@chromium.org>
Reviewed-by: Prashanth Balasubramanian <beeps@chromium.org>
Tested-by: Prashanth Balasubramanian <beeps@chromium.org>
diff --git a/artifact_info.py b/artifact_info.py
index f9ba605..f72c57b 100644
--- a/artifact_info.py
+++ b/artifact_info.py
@@ -51,6 +51,8 @@
 # Tarball containing debug symbols for the given build.
 SYMBOLS = 'symbols'
 
+# The factory test image.
+FACTORY_IMAGE = 'factory_image'
 
 # In general, downloading one artifact usually indicates that the caller will
 # want to download other artifacts later. The following map explicitly defines