apache-kafka service: change default brokerId to -1

A default of 0 means that if you deploy two NixOS boxes with the default
configuration, the second will fail because the brokerId was already in
use. Using -1 instead tells it to pick one automatically at first start.

nixos/modules/services/misc/apache-kafka.nix

@@ -38,7 +38,7 @@ in {
 
     brokerId = mkOption {
       description = "Broker ID.";
-      default = 0;
+      default = -1;
       type = types.int;
     };