CBQE-0: add timeout for large scale tests

Change-Id: I872ec51904099990616191e4be1e2f41ffec49c8
Reviewed-on: http://review.couchbase.org/25898
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: buildbot <build@couchbase.com>