simplify code related to list of candidate Machines

Thomas Morinrequested to merge
simplify-machine-lists into main

This is a follow up to !99 (merged) to do a code simplification suggested by @feleouet

(the code can't be simplified as much as @feleouet expected, because the two lists are used for different purposes, but still the code is nicer after this refactoring, using CAPI collections to select MD or CP machines in selectMachineToDrain)

Edited by Thomas Morin

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