Cluster deletion should be allowed at any time
The user should have the ability to delete a cluster at any time. Currently, we can abort a provisioning cluster, but we cannot delete it until the current task is running. It should be possible to issue a delete at any time, that takes effect immediately.
Remove all pending cluster tasks from queue/planner/solver
Add delete tasks to the queue
Tell provisioner to abort current task (stretch... needs support for this)