Add support for Slurm to use the startup_script module

[nick-stroud]

Allows for passing of startup_script to Slurm V4 using the use keyword. Modules only fall back to this behavior if the other startup script variables are not explicitly set (ex: compute_startup_script)

The controller module takes startup scripts for both compute and controller. I elected to not fallback to the startup_script variable unless neither were set. This is to avoid confusion where startup_script is being only partially overwritten.

Tested:

• Ran ghpc create community/examples/spack-gromacs.yaml --vars project_id=test and verified in generated terraform that in main.tf, slurm_login contained startup_script = module.spack-startup.startup_script.
• Ran integration tests that contains spack-growmacs example.

Submission Checklist

• Have you installed and run this change against pre-commit? (pre-commit install)
• Are all tests passing? (make tests)
• Have you written unit tests to cover this change?
• Is unit test coverage still above 80%?
• Have you updated all applicable documentation?
• Have you followed the guidelines in our Contributing document?

[tpdownes]

Please wait for the full integration checks to go through.