需要通过环境变量支持采集k8s上的rocketmq集群

**Is your feature request related to a problem? Please describe.**
rocketmq集群部署在k8s, 需要采集指标数据.

**Describe the solution you'd like**
rocketmq-exporter容器镜像需要支持环境变量实现动态采集k8s集群上的nameserver

**Describe alternatives you've considered**
可以增加k8s部署rocketmq-exporter的说明文档

**Additional context**
Add any other context or screenshots about the feature request here.