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

Thanks Seth!
I added a small hack to make the id as USN-XXXX-YY until we address anything in the USN DB itself.

> (Grabbing only twenty LSNs at a time, from the web server, seems likely to be brittle some day. We may not be able to do any better if this is what the web team has given us.)

The idea is to get 20, as this is the default from the search page, more than it makes the request to take a longer time to get an answer, if any at all, as sometimes it just times out. And in the request I make, it is in newest order, which make us only get the latest one and it stops requesting whenever it hits an LSN that is already in the JSON file.