Use non-existent channel to default ctest to use own image in update testing.

BUG=chromium-os:19471
TEST=Unittest + ctest by itself to see image used correctly

Change-Id: I17d8ac8d5ec909009dc91593b77b1f65112a88ed
Reviewed-on: http://gerrit.chromium.org/gerrit/6446
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: Chris Sosa <sosa@chromium.org>
diff --git a/buildbot/cbuildbot_commands.py b/buildbot/cbuildbot_commands.py
index 83b543e..bb4439e 100644
--- a/buildbot/cbuildbot_commands.py
+++ b/buildbot/cbuildbot_commands.py
@@ -327,7 +327,7 @@
   if full:
     cmd = ['bin/ctest',
            '--board=%s' % board,
-           '--channel=dev-channel',
+           '--channel=use-local-image',
            '--zipbase=http://chromeos-images.corp.google.com',
            '--type=vm',
            '--no_graphics',