Check if we have tried, pause if we have

For https://github.com/jenkinsci/docker-plugin/issues/36

The launch in the SSH Launcher is catching everything (https://github.com/jenkinsci/ssh-slaves-plugin/blob/master/src/main/java/hudson/plugins/sshslaves/SSHLauncher.java#L667) so we aren't actually able to catch anything ourselves here.  However, it does appear that /somewhere/ there is logic for the ssh launch to retry.  From what I observed, it doesn't always do this and I didn't see any sort of thing in the SSH Launcher, so I guess it might be somewhere related to when Jenkins is actually creating the job.  But anyways, it does appear to keep the same instance of the object when it does retry the launch, so I was able to just create a flag to check if we have tried previously or not.