Support mechanism to provide external zookeeper-server list to build global/configuration zookeeper

[rdhabalia]

Fixes #268

Motivation

Right now, chart dynamically creates zk cluster with zk pods initialized in the same namespace. However, for global/configuration zookeeper, user requires to build zk clusters with pods deployed in different namespaces. Therefore, user needs a mechanism to pass an external list of zk-servers to the chart and build zk-cluster with pods across different namespaces.

Modification

• Chart should be considering zk-value's configuration for external zookeeper and generate zk-configuration file with appropriate zk-server list and unique id of that zookeeper.

This PR sets ZOOKEEPER_SERVERS value provided by user and also sets override-value flag which will be used by generate-zookeeper-config.sh to override external zk list in config file and assign appropriate id to the host.

https://github.com/apache/pulsar/pull/15987 fixes generate-zookeeper-config.sh changes.

Result

• User can add ZOOKEEPER_SERVERS string into zookeeper.configData in Values.yaml file to override external zk-server list.

[rdhabalia]

ping @MarvinCai @michaeljmarshall

[michaeljmarshall]

I missed a minor detail with how the feature is configured. I proposed a fix here https://github.com/apache/pulsar-helm-chart/pull/308.