CBQE-1942: implement downgrade tests

Change-Id: Ic03060eaa164297cfa0151278b95d9c756da770e
Reviewed-on: http://review.couchbase.org/33462
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>