Expose hdparm settings in charm-ceph-osd

[teluka]

There is a big benefit to disabling write cache through hdparm when deploying OSDs on SSD drives (Intel D3-S4610 SSD, hdparm -W 0 /dev/sdX, 7k iops to 22k iops).

I have searched through the ceph-osd charm source code and found that the charm uses hdparm already with the following charm helper function.

https://opendev.org/openstack/charm-ceph-osd/src/branch/master/lib/charms_ceph/utils.py#L471

Those parameters are not available via the juju configuration layer.

It would be beneficial for the I/O performance if the charm’s code could recognize SSD and apply associated tuning settings that would be configurable via charm parameters.