[refactor] use singularity instances
Instances in singularity are basically the same as being able to create an empty container and then run something in it, so we can have the same thing as with nodeos. One main issue with the code as is: we should probably refactor the container_manager code to modify the manifest on the fly according to the options listed in it. It would make it easy to pass along stuff like PWD, necessary binds and so on.
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