MB-100: add rebalance to load_memory test

Change-Id: Ia4e8e36e71487cca22aee5fbf5f3e6b8e68cb3a9
Reviewed-on: http://review.couchbase.org/25150
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>