private void putInNodeCache(String key, Map<String, Set<InstanceType>> cache,
    InstanceType instance) {
    Set<InstanceType> instanceSet = cache.get(key);
    if (instanceSet == null) {
      instanceSet = new HashSet<>();
      instanceSet.add(instance);
    }
    cache.put(key, instanceSet);
  }
Get the ensemble server addresses from the configuration. The format is: host1:port,
host2:port..

@param conf configuration
llap-client/src/java/org/apache/hadoop/hive/registry/impl/ZkRegistryBase.java