Add support for cancelation; stopping an experiment setup early, instead of
waiting till it finished setting up (or fails). This is really nice when a 1000 node experiment has gone awry and it is pointless to wait for it to finish. When we do this, we mark the instance as canceled in the DB, and then wait for create_instance() to notice it. When it does, it stops waiting and invokes terminate with a new cancel option at the backend.
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