Terminate preseed joiners when initiator exits

https://github.com/canonical/microcloud/issues/520 was describing the same situation when forming a MicroCloud interactively.
For preseed it would also be convenient to notify the joiners about a premature exit of the initiator.

Start by reading issue 520, which describes the equivalent interactive-formation behavior, then trace the preseed join flow and how the initiator exit is currently handled. Done means joiners are notified and terminated when the preseed initiator exits prematurely, with coverage for that scenario if the existing flow has tests.

Written by the indexing model from the issue text.