[autotest] Fix import order in label_cleaner

MySQLdb is installed via build_externals on prod machines. This means
that it is only available after 'import common'

BUG=chromium:753134
TEST=Run label_cleaner locally.

Change-Id: Id91b8cbdc718c517720b5e2bc5ddef7f47d7f334
Reviewed-on: https://chromium-review.googlesource.com/604723
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
diff --git a/site_utils/label_cleaner.py b/site_utils/label_cleaner.py
index e1c33d8..d37c67b 100755
--- a/site_utils/label_cleaner.py
+++ b/site_utils/label_cleaner.py
@@ -23,12 +23,13 @@
 
 import argparse
 import logging
-import MySQLdb
 import socket
 import sys
 import traceback
 
 import common
+# Installed via build_externals, must be after import common.
+import MySQLdb
 from autotest_lib.client.common_lib import global_config
 from autotest_lib.client.common_lib import logging_config
 from autotest_lib.server import frontend