Add ZooKeeper Ensemble Support in Master Nodes

[zhangpengshan]

In guagua 0.6.0, zookeeper instance can be started as child process or thread in master node. But still single point failure for such case.

To support multiple masters set by users and worker as active-standby. Start ensemble zookeeper in all master znodes.

Refer to this 0.6.0 issue: #14