CBQE-0: minor fixes for py-memorysanitytests

see MB-11976 and fixed related 134 'Temporary failure' with heavy dgm scenario

Change-Id: I52b60221bfb75d0740a03dd6de4ec02849716089
Reviewed-on: http://review.couchbase.org/40887
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>