tast: Ignore devservers on different subnets. am: 58886d50f3

Change-Id: I4e0193d73d44a5e448b610070fef24a462a64ab7
diff --git a/server/site_tests/tast/tast.py b/server/site_tests/tast/tast.py
index 2a93483..e215ee6 100644
--- a/server/site_tests/tast/tast.py
+++ b/server/site_tests/tast/tast.py
@@ -262,7 +262,7 @@
         The result is saved as self._devserver_args.
         """
         devservers, _ = dev_server.ImageServer.get_available_devservers(
-            self._host.hostname)
+            self._host.hostname, prefer_local_devserver=True)
         logging.info('Using devservers: %s', ', '.join(devservers))
         self._devserver_args = ['-devservers=%s' % ','.join(devservers)]