Change the N1 score to percent of N1 failures

Since for a very many N+1 failures in a cluster, we could actually
degrade the N1 CV by making a node N+1 compliant, we need to make sure
this value only decreases when fixing non-compliant nodes.

The easiest way is to compute the N+1 score as a percentage of failed
nodes, with the caveat that the domain of values might not be fully
compatible with the other scores. It is still [0, 1] but does not vary
like the others.