qa/suites/rados: use default objectsize for upgrade tests

qa/suites/rados: use default objectsize for upgrade tests

in pre-nautilus release, rados cli does not accept `-O` option, so we
should not pass `-O` to this tool. otherwise we will have following
failure:
```
2020-05-01T05:47:04.863 INFO:tasks.radosbench.radosbench.2.smithi183.stderr:unrecognized command -O; -h or --help for usage
2020-05-01T05:47:04.865 DEBUG:teuthology.orchestra.run:got remote process result: 1
```

this change is not cherry-picked from master. as we don't perfor
upgrade tests from pre-nautilus releases

Signed-off-by: Kefu Chai <kchai@redhat.com>