Dispose machines when cloud configuration disappears.

When cloud is renamed/deleted, disposable does not longer locate the connection and are kept until deleted manually. Until we have a way to persist the credentials so we do not have to lookup the cloud itself, better just consider the resources disposed.

```
java.lang.IllegalArgumentException: rhos is not an OpenStack cloud: null
at jenkins.plugins.openstack.compute.JCloudsCloud.getByName(JCloudsCloud.java:184)
at jenkins.plugins.openstack.compute.internal.DestroyMachine.dispose(DestroyMachine.java:50)
at jenkins.plugins.openstack.compute.JCloudsSlave$RecordDisposal.dispose(JCloudsSlave.java:196)
at org.jenkinsci.plugins.resourcedisposer.AsyncResourceDisposer$WorkItem.run(AsyncResourceDisposer.java:286)
at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
```