MB-100: wrong log location in online_upgrade_swap_rebalance

Change-Id: I2dee2a5b27066e4dae237823e9a52d6d30c249b9
Reviewed-on: http://review.couchbase.org/25259
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>