Add additional validation to swarming v1 distributor.

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

Index: dm/api/distributor/swarming/v1/config.proto
diff --git a/dm/api/distributor/swarming/v1/config.proto b/dm/api/distributor/swarming/v1/config.proto
index 1e5288267d231de9750866ce15806444d21fed05..9916dcae07517c607af2602a0c0cb9b84042457b 100644
--- a/dm/api/distributor/swarming/v1/config.proto
+++ b/dm/api/distributor/swarming/v1/config.proto
@@ -10,13 +10,13 @@ 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;