Revisit default mod_cluster metric for WildFly 8

Since the threading aspects of Undertow in WildFly are completely different from JBoss Web's in the previous versions of AS, we should reconsider the default metric for WildFly.

Currently, busyness requires explicit capacity to be useful which would require further tweaking by users.

Metric "cpu" is also a good candidate since it doesn't need explicit capacity..