Make expired simulations checker use running simulations from the Platform.

Context

Simulations weren't schedule for termination because there was no process that checked that simulations actually ran out of time.

Change

This MR makes the process in charge of checking the expired simulations use a single platform.

Other information

This will probably change once we merge this with the multi-region support branch.

EDIT: This is working! I've run a simulation with 100 max sim seconds and it has been automatically shutted down.

Additional documentation