关于emqtt 集群的配置，配置文件中的配置项，都是指什么意思，怎样填写。
Cluster with multicast
cluster.mcast.addr = 239.192.0.1
cluster.mcast.ports = 4369,4370
cluster.mcast.iface = 0.0.0.0
cluster.mcast.ttl = 255
cluster.mcast.loop = on
cluster.discovery = dns
Cluster with DNS
cluster.dns.name = localhost
cluster.dns.app = ekka
cluster.discovery = etcd
Cluster with Etcd
cluster.etcd.server = http://127.0.0.1:2379
cluster.etcd.prefix = emqcl
cluster.etcd.node_ttl = 1m
cluster.discovery = k8s
Cluster with k8s
cluster.k8s.apiserver = http://10.110.111.204:8080
cluster.k8s.service_name = ekka
Address Type: ip | dns
cluster.k8s.address_type = ip
The Erlang application name
cluster.k8s.app_name = ekka
