CBQE-1136: fix Errno 104 for rebalanceout tests

happens only for the last step, when rebalance out 2-1 nodes

Change-Id: I9a305c198d89b052bc3e17c993a30e875fac518c
Reviewed-on: http://review.couchbase.org/25538
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>