Add ZOOKEEPER_HOSTS as an optional property (#5435)

* Add ZOOKEEPER_HOSTS as an optional property

* Check if zookeeper hosts are empty

* Apply scalaFmt

core/invoker/src/main/scala/org/apache/openwhisk/core/invoker/Invoker.scala

@@ -93,6 +93,8 @@ object Invoker {
       kafkaHosts ++
       wskApiHost
 
+  def optionalProperties = zookeeperHosts.keys.toSet
+
   def initKamon(instance: Int): Unit = {
     // Replace the hostname of the invoker to the assigned id of the invoker.
     val newKamonConfig = Kamon.config
@@ -128,7 +130,7 @@ object Invoker {
     }
 
     // load values for the required properties from the environment
-    implicit val config = new WhiskConfig(requiredProperties)
+    implicit val config = new WhiskConfig(requiredProperties, optionalProperties)
 
     def abort(message: String) = {
       logger.error(this, message)(TransactionId.invoker)
@@ -186,7 +188,8 @@ object Invoker {
 
       // --uniqueName is defined with a valid value, id is empty, assign an id via zookeeper
       case CmdLineArgs(Some(unique), None, _, overwriteId) =>
-        if (config.zookeeperHosts.startsWith(":") || config.zookeeperHosts.endsWith(":")) {
+        if (config.zookeeperHosts.startsWith(":") || config.zookeeperHosts.endsWith(":") ||
+            config.zookeeperHosts.equals("")) {
           abort(s"Must provide valid zookeeper host and port to use dynamicId assignment (${config.zookeeperHosts})")
         }
         new InstanceIdAssigner(config.zookeeperHosts).setAndGetId(unique, overwriteId)