CBQE-1206 support rebalance, failover, add_back N nodes

User don't need to mention the ips for cluster topology change anymore
for example, we can define:
    "phases" : {
                "0" :
                {
                    "name" : "swap orchestrator node",
                    "cluster": {"add": "1", "rm": "1", "orchestrator": "True"},
                },
                "1" :
                {
                    "name" : "auto failover and add back",
                    "cluster": {"auto_failover": "2", "add_back": "1"},
                }
}
However, it is also supported hard coded ip format.

Change-Id: I3936b87485eaa74a5dd977bfc2c7475a53ba0ef8
Reviewed-on: http://review.couchbase.org/25645
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>