[FEATURE] Add Exostellar support

[cartalla]

Is your feature request related to a problem? Please describe. Exostellar provides a nested virtualization solution on EC2 that predicts spot terminations far enough in advance to live migrate the instance to another spot or on-demand instance. This enables running long-running, stateful jobs on spot without losing job progress when a spot termination occurs.

Describe the solution you'd like Exostellar support the Slurm scheduler. At a minimum, add documention on how to integrate Exostellar into this Slurm cluster. Ideally, install and configure the software.