MB-100: minor fix in rebalance_in_with_ddoc_compaction

File "pytests/rebalance/rebalancein.py", line 364, in rebalance_in_with_ddoc_compaction
    result = active_task.result()
AttributeError: 'list' object has no attribute 'result'

Change-Id: I4e460a30fadeaa1c7d0c1092eb3b06de3893d89a
Reviewed-on: http://review.couchbase.org/24505
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>