MB-100: minor fixes in upgrade tests

Change-Id: Id3c82949af918a82ddd324594c673f628a3a9b06
Reviewed-on: http://review.couchbase.org/24472
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>