devserver: Download the autotest_server_package.

Adds support for the autotest server package to the devserver.
The tarball is downloaded and not extracted.

BUG=chromium:455344
TEST=http://localhost:8080/stage?archive_url=gs://chromeos-image-archive/
trybot-peppy-release/R42-6753.0.0-b301&artifacts=autotest_server_package

Change-Id: If84b29086d77f4000095ee42c882d5e6114aff8e
Reviewed-on: https://chromium-review.googlesource.com/246251
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
diff --git a/artifact_info.py b/artifact_info.py
index 712b287..c1052d2 100644
--- a/artifact_info.py
+++ b/artifact_info.py
@@ -46,6 +46,9 @@
 # Autotest Packages-- the autotest packages subdirectory.
 AUTOTEST_PACKAGES = 'autotest_packages'
 
+# Autotest Server Package.
+AUTOTEST_SERVER_PACKAGE = 'autotest_server_package'
+
 # Test Suites - just the test suites control files from the autotest directory.
 TEST_SUITES = 'test_suites'