mesos service: only pass --zk if quorum > 0

The ZooKeeper URL is not necessary when using in-memory slave
registration.

nixos/modules/services/misc/mesos-master.nix

@@ -85,8 +85,9 @@ in {
         ExecStart = ''
           ${pkgs.mesos}/bin/mesos-master \
             --port=${toString cfg.port} \
-            --zk=${cfg.zk} \
-            ${if cfg.quorum == 0 then "--registry=in_memory" else "--registry=replicated_log --quorum=${toString cfg.quorum}"} \
+            ${if cfg.quorum == 0
+              then "--registry=in_memory"
+              else "--zk=${cfg.zk} --registry=replicated_log --quorum=${toString cfg.quorum}"} \
             --work_dir=${cfg.workDir} \
             --logging_level=${cfg.logLevel} \
             ${toString cfg.extraCmdLineOptions}