CBQE-0: clear logs in newupgradetests.py

Change-Id: I14de3993b525326268643feaf2d09ff245c6e2d2
Reviewed-on: http://review.couchbase.org/30307
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>