This adds the 'db_seed' management command (python3 manage.py db_seed)
for automatically seeding the db with default entries.

- initial docker OS list (debian, centos, fedora, ubuntu)
- default job queue (named 'default')
- the 'inria' Group

Additionally, in the dev setup, it creats 3 users (all with
password 'allgo'):
- admin@localhost which is superuser
- devel@localhost which own a webapp named 'sleep'
- guest@localhost