Provide snapshot hook for daily_checkpoint.sh.
Perhaps the most common SDP customization is a tweak to integrate
the SDP checkpoint mechanism with 'Snapshot' capability of the
underlying hardware, so that the snapshot is created at the ideal
point in time, right after the metadata checkpoint is created.
The intent of this change is to reduce the need for such
customization by making a well defined hook in the daily checkpoint
process for calling a site-specific custom checkpoint script.