Add additional validation to swarming v1 distributor.

dm/api/distributor/swarming/v1/config.proto

 // Copyright 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";
 
 import "google/protobuf/duration.proto";
 
 package swarmingV1;
 
 message Config {
   message Swarming {
-    string host = 1;
+    string url = 1;
   }
   Swarming swarming = 1;
 
   message Isolate {
     // The host of the isolate service, e.g. isolateserver.appspot.com.
-    string host = 1;
+    string url = 1;
 
     // The default assumed expiration period for execution results.
     google.protobuf.Duration expiration = 2;
   }
   Isolate isolate = 2;
 }