Milo: Use luci-config for storing buildbot acls

milo/common/config/settings.proto

+// Copyright (c) 2016 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+syntax = "proto3";
+
+package config;
+
+// Settings represents the format for the global (service) config for Milo.
+message Settings{
+  message Buildbot{
+    // internal_reader is the infra-auth group that is allowed to read internal
+    // buildbot data.
+    string internal_reader = 1;
+  }
+  Buildbot buildbot = 1;
+}