Provide an executor to run slurm locally

[SvenMarcus]

Currently HPC Rocket only runs slurm commands over SSH. However, if the CI runner is directly running on a machine that has slurm installed, it should also be able to run these commands locally.

The abstract class CommandExecutor already provides us with the necessary interface to substitute the SSHExecutor with a local one. Some additional arguments may be required to choose the correct executor and filesystem at runtime.