void shutdown() {
        if (zooKeeperRunner != null) {
            zooKeeperRunner.shutdown();
        }
    }
Starts zookeeper server and supports reconfiguring zookeeper cluster. Keep this as a component
without injected config, to make sure that it is not recreated when config changes.

@author hmusum
zookeeper-server/zookeeper-server-common/src/main/java/com/yahoo/vespa/zookeeper/Reconfigurer.java