MB-11017: fix RebalanceTask with single node

Change-Id: I0f3d58a71cd7a60c47e661fe0f90ab6ba1448522
Reviewed-on: http://review.couchbase.org/36697
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: buildbot <build@couchbase.com>