Readahead for use_existing

When using use_existing, readahead is not set on the OSDs because they do not have the labels CBT expects. We should probably determine which devices are mounted in /var/lib/osd/\* and then set their readahead values.

Start by tracing the use_existing path and how CBT currently finds devices through its expected labels. Inspect how devices mounted under /var/lib/osd/* can be identified, then verify that readahead is set for those OSD devices; done means existing OSDs receive the intended readahead values without CBT labels.

Written by the indexing model from the issue text.