Remove metrics-ganglia from default build due to LGPL issue

The metrics ganglia code depends on an LGPL library which we can't distribute with Spark. More information can be found here:
https://groups.google.com/forum/#!searchin/metrics-user/lgpl/metrics-user/1tQd_qZHQNE/TqAfXYwh7OUJ

We should isolate this code in a separate module inside of an `/extras` folder and have a build flag in Maven/SBT (e.g. `-Pganglia`) that will pull this in if desired. That way users can still use it if they do a special build.