CBQE-1220 Fix bug about warmup test case with reboot operation

Change-Id: I0ed4aa264013f63f23f56d26b50faf78f86cda33
Reviewed-on: http://review.couchbase.org/26414
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Chisheng Hong <chisheng@couchbase.com>