SLING-3251 - Enable Logback ChangeLevelDispatcher by default if JUL Integration is enabled

Added a check if LevelChangePropagator is installed or not (through config) in case bridge handler is enabled. If not then one would be installed

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1551120 13f79535-47bb-0310-9956-ffa450edef68