Ivy Plugin change

[Burghduff]

I've submitted a pull request here https://github.com/jenkinsci/ivy-plugin/pull/18. We were having issues that when our slaves were offline during startup, the Ivy plugin would look at the builds on the master and then try to find a workspace on a node that was not available.  My change just checks to see if the node is available and if not, it attempts to use the ivy backup file that is stored with the project on the master.  I am not sure who the maintainer is for this project.

Can someone give this a quick review?

Thanks for you time