E2e test planning

[alicefr]

Let's discuss about the test planning. Currently, we only test the happy path for the reboot

Ideas about follow-up tests:

1. Extend the test TestUpdateReboot by rolling back to the previous image
2. Pause the rollout and check that the resuming continue correctly
3. Try to upgrade to no-existing image, the node should go in degraded state and the rollout should be paused
4. Shutdown (manually) a node after the reboot to simulate a faulty node, after the upgrade. It should be reported as degraded
5. Try to kill the controller/daemon during a roll-out. They should recover and the rollout should progress and terminate successfully
6. Testing the happy path with the digests (once supported)
7. Upgrade to an image protected by a pull secret (once supported)