CBQE-1131 automate the phase failover node by killing memcached, add back and rebalance

The test definition can be:
    "phases" : {
                "0" :
                {
                    "name" : "autofailover_addback_rebalance",
                    "cluster": {"auto_failover":"10.6.2.38", "add_back": "10.6.2.38"}
                }
}

Change-Id: I047264a5554d8e64c4f3429e2f8a0d85e8391437
Reviewed-on: http://review.couchbase.org/25269
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>
Tested-by: Chisheng Hong <chisheng@couchbase.com>