autotest: move constants to common_lib

This file gets imported from lots of server-side tests and
infrastructure so it shouldn't be hiding in client/cros. This
moves the file and fixes all imports.

BUG=chromium:485834
TEST=run many tests, make sure there are no problems importing
  constants

Change-Id: I5c2b2a0bef6ebc998e62f40162a6ec9cc4436480
Reviewed-on: https://chromium-review.googlesource.com/291302
Trybot-Ready: Eric Caruso <ejcaruso@chromium.org>
Commit-Queue: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Chris Sosa <sosa@google.com>
diff --git a/frontend/apache_auth.py b/frontend/apache_auth.py
index 8971f43..0e9d218 100644
--- a/frontend/apache_auth.py
+++ b/frontend/apache_auth.py
@@ -3,7 +3,7 @@
 from django.contrib import auth
 from django import http
 
-from autotest_lib.client.cros import constants
+from autotest_lib.client.common_lib.cros import constants
 from autotest_lib.frontend import thread_local
 from autotest_lib.frontend.afe import models, management
 from autotest_lib.server import utils