Linear raid can be created using `mdadm --create --level=linear`,
By creating a linear raid disk on top of a logical volume,
we can pass the linear device to `ceph-volume lvm batch --prepare`
to provision a new OSD on top of the logical volume since
`ceph-volume` does not take lv as the data device.
Signed-off-by: Ash Wu <hSATAC@gmail.com>