Add additional validation to swarming v1 distributor.

dm/api/distributor/swarming/v1/params.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";
 
 import "github.com/luci/luci-go/dm/api/distributor/swarming/v1/isolate_ref.proto";
 
@@ skipping 48 matching lines @@
       repeated swarmingV1.IsolatedRef isolated = 1;
 
       message CipdPackage {
         string name = 1;
         string path = 2;
         string version = 3;
       }
       // 0 or more cipd packages for the job.
       repeated CipdPackage packages = 2;
 
-      // Optional server to use for cipd packages. If you know you're using
-      // swarming's default server, you can omit this.
+      // Server to use for cipd packages. Required (though it may be specified
+      // by the quest's designated distributor config). This must be
+      // scheme://<hostname>, e.g. "https://example.appspot.com". The only
+      // supported schemes are http and https.
       string cipd_server = 3;
     }
     Inputs inputs = 1;
 
     // This is the "argv" to run with this job. This includes substitution
     // paramters defined by swarming's run_isolated.py script:
     //   https://github.com/luci/luci-py/blob/master/client/run_isolated.py
     //
     // Additionally, DM provides the following substitutions:
     //   ${DM.PREVIOUS.EXECUTION.STATE:PATH} - the path to a JSONPB encoding of
     //     the swarming_v1.Result from the previous Execution of this Attempt.
     //   ${DM.QUEST.DATA.DESC:PATH} - the path to the
     //     dm.Quest.Data.Desc JSONPB for the quest that this execution is part
     //     of.
     //   ${DM.EXECUTION.AUTH:PATH} - the path to the JSONPB of a
     //     dm.Execution.Auth for the current execution. Consuming this file is
     //     currently required to interact with DM, making this substitution
     //     parameter a requirement.
     //   ${DM.HOST} - the hostname to use to access DM's pRPC API.
     //
     // Command MUST be specified; specifying a command in any of the isolated
     // inputs WILL NOT DO ANYTHING.
     repeated string command = 2;
 
     // Environment variables for the executions.
     map<string, string> env = 4;
   }
   Job job = 3;
 }