fio: add iostat and keep the output files

[Vicente-Cheng]

• that we can do more investigation with these output files

For example, we will get the output like the following.

harvester-node-2:~ # ls output/
Local-Path-bandwidth-iostat.log  Local-Path-iops.json           Local-Path_vs_Longhorn.summary  Longhorn-iops-iostat.log     Longhorn-latency.json
Local-Path-bandwidth.json        Local-Path-latency-iostat.log  Longhorn-bandwidth-iostat.log   Longhorn-iops.json
Local-Path-iops-iostat.log       Local-Path-latency.json        Longhorn-bandwidth.json         Longhorn-latency-iostat.log

We could compare iostat data between Local-Path and Longhorn for further investigation.