Appoint every task of the taskCollection of the RegionAllocator to a
region and return a list of regionWorkloadCollections, which is a
collection of tasks associated with a certain geographical region.
This class is used to represent the different options used during the
resource allocation phase of the broker: only using on-demand instances is
the first option, no spot instances is another option, spot-enabled (which
uses the spot model to determine the bid) and spot-optimal (which used the
actual spot price history to determine the bid).