SLING-3242 - Allow referring to OSGi appenders from within Logback config

Adding a new action OsgiAppenderRefAction which allows referring to OSGi appenders
from within the config file. The logger -> appender mapping would be stored in
AppenderTracker and it would associate the appender to all such logger whenever
such a service is registered

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1541422 13f79535-47bb-0310-9956-ffa450edef68