[FEATURE] Add support for nss_slurm plugin

[cartalla]

• Add support for nss_slurm plugin
  • https://slurm.schedmd.com/nss_slurm.html
  • Removes the need of the user_groups.json file and creating local users and groups.
  • 1/21/22: Implemented but when I remove the local user the username isn't found inside the job. The uid and gid exist, but without names. This results in "I have no name" in an interactive shell. May be acceptable to some, but sticking with the previous method of creating users and groups.