Fix resource reservation

Trying to keep the same codepath for both watermark and standard modes is proving to be a bit more difficult than expected. This change addresses an issue with the resource reservations and the logic we iterate over the hosts. The watermark mode is best effort and we need to gather the requested resources even if we cannot find all of them. On the other hand, in the standard mode we only preempt servers if all the requested resources are found. Reservations should be considered only for the standard mode, because in the watermark mode we loop over the same once at most.