Milo: Fix naming of pubsub settings

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;
 
-    // public_topic is the name of the pubsub topic where public builds come in
+    // public_subscription is the name of the pubsub topic where public builds come in
     // from
-    string public_topic = 2;
+    string public_subscription = 2;
 
-    // internal_topic is the name of the pubsub topic where internal builds
+    // internal_subscription is the name of the pubsub topic where internal builds
     // come in from
-    string internal_topic = 3;
+    string internal_subscription = 3;
   }
   Buildbot buildbot = 1;
 
   message Buildbucket {
     // name is the user friendly name of the Buildbucket instance we're pointing to.
     string name = 1;
 
     // host is the hostname of the buildbucket instance we're pointing to (sans schema).
     string host = 2;
   }
   Buildbucket buildbucket = 2;
 
   message Swarming {
     // default_host is the hostname of the swarming host Milo defaults to, if
     // none is specified.  Default host is implicitly an allowed host.
     string default_host = 1;
 
     // allowed_hosts is a whitelist of hostnames of swarming instances
     // that Milo is allowed to talk to.  This is specified here for security
     // reasons, because Milo will hand out its oauth2 token to a swarming host.
     repeated string allowed_hosts = 2;
   }
   Swarming swarming = 3;
 }