Add additional validation to swarming v1 distributor.

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

Index: dm/api/distributor/swarming/v1/params.proto
diff --git a/dm/api/distributor/swarming/v1/params.proto b/dm/api/distributor/swarming/v1/params.proto
index dff6dd121026f58eb6ecb886cd223ba1e532eaf3..e806d807c7dd0d6c0dbcebf55f4e0d92b71c2a93 100644
--- a/dm/api/distributor/swarming/v1/params.proto
+++ b/dm/api/distributor/swarming/v1/params.proto
@@ -66,8 +66,10 @@ message Parameters {
       // 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;