test/crimson/cbt: increase stddev variance to 2.0

test/crimson/cbt: increase stddev variance to 2.0

as per Mark requests. we've being observing

```
03:55:21 - WARNING  - cbt      - prefill/incerta04.front.sepia.ceph.com/1: iops_stddev: (or (less) (near 0.50)):: 434.037/69.2122  => rejected
...
04:37:52 - WARNING  - cbt      - rand/incerta04.front.sepia.ceph.com/0: iops_stddev: (or (less) (near 0.50)):: 3830.51/2473.17  => rejected
...
04:37:52 - WARNING  - cbt      - rand/incerta04.front.sepia.ceph.com/1: iops_avg: (or (greater) (near 0.05)):: 53916.0/57375.0  => rejected
```

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