WorkspaceCleanupThread: the workspace thread is disabled by default

The workspace cleanup feature should be off by default. It creates a lot of FUD(fear, uncertainty and doubt) when Jenkins starts falling over for no apparant reason after having worked smoothly for months.

With this change, the bug below can be closed because users who don't want this won't be hurt by this feature.

https://issues.jenkins-ci.org/browse/JENKINS-9436
Changing the name of the property is going to break existing deployments that use it. This change is about changing its default value.