'Invalidate Generated Certificate by  marking it \'unavailable\'.
Following is the list of fields with their defaults
1 - verify_uuid = \'\',
2 - download_uuid = \'\',
3 - download_url = \'\',
4 - grade = \'\'
5 - status = \'unavailable\''

def invalidate(self):

    self.verify_uuid = ''
    self.download_uuid = ''
    self.download_url = ''
    self.grade = ''
    self.status = CertificateStatuses.unavailable
    self.save()
