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 
__ is not as good as dummy0 for making clear that a) we're throwing this value away and b) we don't want pylint to complain. This is a totally unnecessary line of diff. You have no idea how much self control it required to limit myself to only changing this.