Select a subset of configs for manifests that are really platform specific

There are some manifests that are really platform specific and could fail differently on many different configurations (maybe because they are big manifests and contain many tests, or maybe because they test platform specific areas of Firefox, e.g. network). Currently we select all configs that the manifest could independently fail on, we could instead select a subset of them, trying to select the most diverse set. We can take into account:

• manifest confidence (the higher it is, the higher number of configs we select);
• config "independence" (if the manfiests fails on both linux and windows 70% of the times, and on mac and linux only 30% of the time, select mac and linux);
• config diversity (if you have to choose two between linux32, linux64, and windows64, select linux64 and windows64).