AllocElement: extend with the cluster score

AllocElement, a type used as a result of allocations, holds the status
of the nodes after the allocation. In most cases, we'll compare this
allocation result with others, to see which allocation decision makes
the most sense. This comparison is done via the cluster score.

However, if we later need to redo this computation, as part of other
comparisons, we'd need to evaluate it again, etc. So it's easier to just
compute the score at the place where we compute the node list in the
initial step.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>