MB-100: fix rebalance thread invocation

Change-Id: Ie26548cd6da3e36d2a500a88f2e6db987d5d891e
Reviewed-on: http://review.couchbase.org/24437
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>