Stanford: Minimal change for LPL Long run

* Adds flags for ungenerated_certs to allow cert runs which retain
  pre-existing certs, even when students fail by some new set of
  criteria, but which recreate certs for passing students, even when
  they previously had one.

@stvstnfrd and @jbau 
The simplest thing that could possibly work. We spend this whole method building up a new cert object, but we only save it back to the database if keep_current is False (which is its default). This *should* retain the cert data that is already extant in the db, without modification.