Support mechanism to provide external zookeeper-server list to build global/configuration zookeeper (#269)

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

* Add external zk example

* add external zk list into values.yaml

Fixes #268

### Motivation
Right now, [chart dynamically](https://github.com/apache/pulsar-helm-chart/blob/master/charts/pulsar/templates/zookeeper-statefulset.yaml#L140) 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](https://github.com/apache/pulsar/blob/master/docker/pulsar/scripts/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](https://github.com/apache/pulsar/blob/master/docker/pulsar/scripts/generate-zookeeper-config.sh) changes.


### Result
- User can add `ZOOKEEPER_SERVERS` string into `zookeeper.configData` in [Values.yaml](https://github.com/apache/pulsar-helm-chart/blob/master/charts/pulsar/values.yaml#L385) file to override external zk-server list.

charts/pulsar/templates/zookeeper-statefulset.yaml

@@ -138,10 +138,19 @@ spec:
           containerPort: {{ .Values.zookeeper.ports.clientTls }}
         {{- end }}
         env:
-        - name: ZOOKEEPER_SERVERS
+         - name: ZOOKEEPER_SERVERS
-          value:
+        {{- if .Values.zookeeper.configData.ZOOKEEPER_SERVERS }}
-            {{- $global := . }}
+           value: {{ .Values.zookeeper.configData.ZOOKEEPER_SERVERS }}
-            {{ range $i, $e := until (.Values.zookeeper.replicaCount | int) }}{{ if ne $i 0 }},{{ end }}{{ template "pulsar.fullname" $global }}-{{ $global.Values.zookeeper.component }}-{{ printf "%d" $i }}{{ end }}
+        {{- else }}
+           {{- $global := . }}
+           value: {{ range $i, $e := until (.Values.zookeeper.replicaCount | int) }}{{ if ne $i 0 }},{{ end }}{{ template "pulsar.fullname" $global }}-{{ $global.Values.zookeeper.component }}-{{ printf "%d" $i }}{{ end }}
+        {{- end }}
+         - name: EXTERNAL_PROVIDED_SERVERS
+        {{- if .Values.zookeeper.configData.ZOOKEEPER_SERVERS }}
+           value: "true"
+        {{- else }}
+           value: "false"
+        {{- end }}
         envFrom:
         - configMapRef:
             name: "{{ template "pulsar.fullname" . }}-{{ .Values.zookeeper.component }}"

charts/pulsar/values.yaml

@@ -397,6 +397,8 @@ zookeeper:
   ## templates/zookeeper-configmap.yaml
   ##
   configData:
+    # External zookeeper server list in case of global-zk list to create zk cluster across zk deployed on different clusters/namespaces
+    # ZOOKEEPER_SERVERS: "us-east1-pulsar-zookeeper-0.us-east1-pulsar-zookeeper.us-east1.svc.cluster.local:2888:3888,us-east1-pulsar-zookeeper-1.us-east1-pulsar-zookeeper.us-east1.svc.cluster.local:2888:3888,us-east1-pulsar-zookeeper-2.us-east1-pulsar-zookeeper.us-east1.svc.cluster.local:2888:3888,us-west1-pulsar-zookeeper-0.us-west1-pulsar-zookeeper.us-west1.svc.cluster.local:2888:3888,us-west1-pulsar-zookeeper-1.us-west1-pulsar-zookeeper.us-west1.svc.cluster.local:2888:3888,us-west1-pulsar-zookeeper-2.us-west1-pulsar-zookeeper.us-west1.svc.cluster.local:2888:3888"
     PULSAR_MEM: >
       -Xms64m -Xmx128m
     PULSAR_GC: >

examples/values-zookeeper-aws.yaml

@@ -45,6 +45,9 @@ monitoring:
   node_exporter: false
 
 zookeeper:
+  configData:
+    # External zookeeper server list in case of global-zk list to create zk cluster across zk deployed on different clusters/namespaces
+    # ZOOKEEPER_SERVERS: "us-east1-pulsar-zookeeper-0.us-east1-pulsar-zookeeper.us-east1.svc.cluster.local:2888:3888,us-east1-pulsar-zookeeper-1.us-east1-pulsar-zookeeper.us-east1.svc.cluster.local:2888:3888,us-east1-pulsar-zookeeper-2.us-east1-pulsar-zookeeper.us-east1.svc.cluster.local:2888:3888,us-west1-pulsar-zookeeper-0.us-west1-pulsar-zookeeper.us-west1.svc.cluster.local:2888:3888,us-west1-pulsar-zookeeper-1.us-west1-pulsar-zookeeper.us-west1.svc.cluster.local:2888:3888,us-west1-pulsar-zookeeper-2.us-west1-pulsar-zookeeper.us-west1.svc.cluster.local:2888:3888"
   volumes:
     # use a persistent volume or emptyDir
     persistence: true