spindle: make workflows engine-agnostic #423

Still needs docs, but other than that should be alright for a first pass.

We may want to make the whole cleanupFuncs thing also in the interface? Not sure, I think I could be swayed either way; a single DestroyWorkflow function is definitely simpler, but would require each engine to do its own tracking of what to actually do to clean up after a given workflow.

Depending on the engine, it doesn’t always have to be functions per-workflow, could just be an API call that’s always run instead of the Docker engine where it does certain things based on what actually succeeded.