allow custom-formatter configuration

Currently only org.jboss.logmanager.formatters.PatternFormatter is supported in JBoss AS7 configuration. It would be very useful to allow specifying a custom-formatter (e.g java.util.logging.XMLFormatter).

At the moment one can specify another formatter in logging.properties but once the logging subsystem is initialized, the settings in logging.properties get overridden.

A workaround would be to disable logging subsystem but that has drawbacks.