Add TEMPLATEFILE parameter for cert-data stanzas

* Adds optional TEMPLATEFILE parameter for cert-data stanzas, which
  allows course-by-course override of the template used for a course.
  This allows the same template to be used for several courses offered
  by a single instructor, for example.

* This is also a small but necessary evolutionary step in the direction
  of fully dynamic cert generation.

@stvstnfrd @caesar2164 and @sarina:

I think we've all talked about this. Among other things, this lets us handle the case where one instructor has many courses without having many copies of the same template file. This is transitional towards a time when instructors can manage their own assets and all the course stuff is magical.

@dianakhuang FYI.