Description of problem:
The place-holders mechanism was added to the hibernate command as a proof of concept. The problem is that this mechanism is wrong: the criteria whether the command can be safely ended as successful after engine restart (once all the tasks are finished successfully) should not be when the number of tasks matches the expected number of tasks. It should be: only when the execute phase ends successfully (just like regular, i.e not engine restart, flow - we poll the tasks after the execute phase is finished).

We are about to change the hibernate command to call add-disk instead of create-image, so we need to remove the place-holders from it. without an alternative we'll suffer from the bug that lead us to the place-holders solution.

I can think of 2 options:

1. to mark tasks which are created as part of hibernate command as tasks that should be ignored after engine restart - it's a nice technical challenge to resume the operation after engine restart, but I'm not sure it is very useful, we can live with failed hibernate commands after engine restart.

2. to mark the tasks as such that should be polled after engine restart only when the execute phase ends successfully.

I recommend on the #1.