Add ability to mark nodes as offline/disconnected.

In our infrastructure we share a Jenkins config between local, dev and production environments. When we add a node we generally want it to only run in production (but we might want to connect to another environement for development and testing).

It would be nice if we could have a flag in JCasC to signify if a node should be marked as disconnected on Jenkins start-up. This would prevent a lot of needless SSH connection attempts (we set invalid credentials at to avoid multiple Jenkins instances running jobs at once).

A more general usecase would be if a node is broken, or needs investigation, it would be nice to store this state in JCasC code, rather than relying on people disconnecting through the UI (and Jenkins not reinitiating connection).