autotest: use cautotest as default web argument for stable_images

Metrics were failing prior to this, due to None value in a metric field.
Fix this by making the argument non-None in the default case (in which
it is not supplied).

BUG=chromium:898934
TEST=None

Change-Id: I1ce36760cfc1907163036b96b234596f13978f86
Reviewed-on: https://chromium-review.googlesource.com/c/1313659
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
diff --git a/site_utils/stable_images/assign_stable_images.py b/site_utils/stable_images/assign_stable_images.py
index 8e4f2c2..7098ea9 100755
--- a/site_utils/stable_images/assign_stable_images.py
+++ b/site_utils/stable_images/assign_stable_images.py
@@ -27,7 +27,6 @@
 
 import argparse
 import logging
-import sys
 
 import common
 from autotest_lib.server.cros.dynamic_suite import frontend_wrappers
@@ -111,7 +110,7 @@
         @param old_default  The original default version.
         @param new_default  The new default version to be applied.
         """
-        logging.debug('Default %s -> %s' % (old_default, new_default))
+        logging.debug('Default %s -> %s', old_default, new_default)
 
     def _report_board_changed(self, board, old_version, new_version):
         """
@@ -368,7 +367,7 @@
     # TODO(crbug/888046) Make these arguments required once puppet is updated to
     # pass them in.
     parser.add_argument('--web',
-                        default=None,
+                        default='cautotest',
                         help='URL to the AFE to update.')
 
     arguments = parser.parse_args()