Invoker crashed due to lack of ZookeeperHost Config

[FangyueLiu123]

Environment details:

• local deployment, native ubuntu, Mac OS, Kubernetes, ...
• version of docker, ubuntu, ...

Steps to reproduce the issue:

1. Deploy the whole stack on k8s, including zookeeper, redis, couchdb and so on.

Provide the expected results and outputs:

invoker should work normally.

Provide the actual results and outputs:

Invoker crashed:
Exception in thread "main" java.lang.IllegalArgumentException: connectionString cannot be null or empty
        at org.apache.curator.shaded.com.google.common.base.Preconditions.checkArgument(Preconditions.java:141)
        at org.apache.curator.ensemble.fixed.FixedEnsembleProvider.<init>(FixedEnsembleProvider.java:55)
        at org.apache.curator.ensemble.fixed.FixedEnsembleProvider.<init>(FixedEnsembleProvider.java:43)
        at org.apache.curator.framework.CuratorFrameworkFactory$Builder.connectString(CuratorFrameworkFactory.java:234)
        at org.apache.curator.framework.CuratorFrameworkFactory.newClient(CuratorFrameworkFactory.java:109)
        at org.apache.curator.framework.CuratorFrameworkFactory.newClient(CuratorFrameworkFactory.java:94)
        at org.apache.openwhisk.core.invoker.InstanceIdAssigner.setAndGetId(InstanceIdAssigner.scala:37)
        at org.apache.openwhisk.core.invoker.Invoker$.main(Invoker.scala:191)
        at org.apache.openwhisk.core.invoker.Invoker.main(Invoker.scala)

Additional information you deem important:

• changes you did or observed in the environment

It seems that invoker crash because config.ZookeeperHosts is null string.
This may be caused by #5375 which remove it from invoker, however, CuratorFrameworkFactory.newClient requires it to be not null when zookeeper is used.

Adding ZookeeperHosts to the requiredProperties solves this bug.

[style95]

I met this problem today.

This is because ZookeeperHosts is removed from the requiredProperties and causes an environment variable not to be transformed into a configuration.
https://github.com/apache/openwhisk/blob/master/common/scala/src/main/scala/org/apache/openwhisk/common/Config.scala#L142

I think we need to add the config back to the requiredProperties for now.

[FangyueLiu123]

Thanks you a lot for solving this.