temporary user created for each test should be cleaned on tierdown

[jhutar]

Every test creates its own user - should that one be deleted on tier down?
Correct. Totally forgot about that.

[abalakh]

Small correction - not every test, but every test module.
Robottelo's delete_session_user for the reference:
https://github.com/SatelliteQE/robottelo/blob/master/robottelo/test.py#L380

Btw issue has nothing to do with airgun itself, as session user gets created and used in robottelo/tests/airgun_poc/conftest.py , but yeah, let's probably track it here for now (as we don't have separate issue tracker for airgun_poc robottelo branch.