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.
Merge pull request #38 from edx/jrbl/template_selection

Add TEMPLATEFILE parameter for cert-data stanzas