Add log level system variables

There is a lot of messages in Hawkular log which developers may not find interesting and in turn many lower level ones that they might want to see when solving a particular task. The present objective is to add system properties to standalone.xml files that will control log levels of logging categories related to Hawkular. There will be appropriate defaults set that can be overridden on command line, e.g. to override the default INFO level for org.hawkular.bus logging category, one can invoke the start script as follows:

standalone.sh -Dhawkular.log.bus=DEBUG