[MICROBA-1100]
* Add CertificationGenerationCommandConfiguration model that will store the command arguments for the `cert_generation` mgmt command
* Add ability to add entries to the CertificationGenerationCommandConfiguration through Django admin
* Update mgmt command with ability to read arguments from the config model/database
* Fix failing test in `test_cert_generation.py`
* Add new test for missing `users` argument in `test_cert_generation.py`