[autotest] Support shadow config for perf_uploader

This change adds the support of a local shadow config for perf_uploader. The
shadow config file should be in the same folder of perf_dashboard_config.json,
and named perf_dashboard_shadow_config.json.

BUG=None
TEST=local run, unittest

Change-Id: I1ff73b6dceabf3234c64ebdcbff665b2487906a3
Reviewed-on: https://chromium-review.googlesource.com/320395
Commit-Ready: Dan Shi <dshi@google.com>
Tested-by: Dan Shi <dshi@google.com>
Reviewed-by: Fang Deng <fdeng@chromium.org>
diff --git a/.gitignore b/.gitignore
index 8d7bf84..d09e866 100644
--- a/.gitignore
+++ b/.gitignore
@@ -69,3 +69,6 @@
 
 # Shadow config for ssp deploy
 ssp_deploy_shadow_config.json
+
+# Shadow config for perf dashboard
+tko/perf_upload/perf_dashboard_shadow_config.json