Introduce per-metric weights

Currently all metrics have the same weight (we just sum them together).
However, for the hard constraints (N+1 failures, offline nodes, etc.)
we should handle the metrics differently based on their meaning. For
example, an instance living on a primary offline node is worse than an
instance having its secondary node offline, which in turn is worse than
an instance having its secondary node failing N+1.

To express this case in our code, we introduce a table of weights for
the metrics, with which we can influence their relative importance.