This command can be used to manage the ZooKeeper cluster for Fabric.
There are four different usages:

* fabric:zk-cluster
Without any arguments, the list of agents composing the current cluster will be displayed.

* fabric:zk-cluster [list of agents]
This mode will create a new cluster composed of the given agents and will switch all agents to this new cluster.

* fabric:zk-cluster --add [list of agents]
This mode will create a new cluster composed of the existing agents and the agents specified on the command line. All agents will be switched to this new cluster.

* fabric:zk-cluster --remove [list of agents]
This mode will create a new cluster composed of the existing agents excluding the agents specified on the command line. All agents will be switched to this new cluster.

Note that given ZooKeeper is the key component of Fabric, changing the ZooKeeper cluster is a critical operation.  All data will be preserved and copied to the new cluster before swichting.
