Certificates: Additional flags for regenerate_user

* -G allows explicit setting of grade condition (e.g., a string such as
  'Distinction', 'Pass', 'F', 'B') for the certificate, regardless of
  grader output for the user.
* -T allows explicit request of a particular certificate template to be
  applied to a given user's certificate regeneration, i.e.:
    -T doctor_distinction.pdf
  to generate a certificate of distinction for doctors.
Merge pull request #2530 from edx/jrbl/certs_regen_handy_flags

Certificates: Additional flags for regenerate_user