Add the 'Discard Old Build ' plugin.

This is for issue #473, initially let's just add the plugin. 
Why have all those lines been removed? Can we use only the log rotator OR discard old build plugin? If that is the case you should really move over the current settings for numToKeep and do not set it to -1. As I said a change to that is a different step for this issue. On this PR we only want to make sure that we do not store successful builds.