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/server/hosts/servo_host.py b/server/hosts/servo_host.py
index bec28ef..5052ca9 100644
--- a/server/hosts/servo_host.py
+++ b/server/hosts/servo_host.py
@@ -20,11 +20,11 @@
 from autotest_lib.client.common_lib import global_config
 from autotest_lib.client.common_lib import lsbrelease_utils
 from autotest_lib.client.common_lib.cros import autoupdater
+from autotest_lib.client.common_lib.cros import constants as client_constants
 from autotest_lib.client.common_lib.cros import dev_server
 from autotest_lib.client.common_lib.cros import retry
 from autotest_lib.client.common_lib.cros.graphite import autotest_stats
 from autotest_lib.client.common_lib.cros.network import ping_runner
-from autotest_lib.client.cros import constants as client_constants
 from autotest_lib.server import site_utils as server_site_utils
 from autotest_lib.server.cros.servo import servo
 from autotest_lib.server.cros.dynamic_suite import frontend_wrappers