Code review comment for ~ebarretto/ubuntu-cve-tracker:oval-lsn

Thanks Alex and Seth,
I took both suggestions into consideration and added a prepend_usn_to_id function using regex, making it easier to adjust or remove it whenever we need.
I've also gave +x to the fetch-lsns script.

I would appreciate a check on both last commits just to make sure I didn't break anything.
Thanks!