System variant: no scheduler

[pearce8]

Enable a way to generate run scripts without scheduler instructions, for interactive runs or composing CI runs.

./bin/benchpark system init --dest=tioga-system tioga rocm=551 compiler=cce ~gtl scheduler=mpi

• [ ] Base class only has the no scheduler instructions option (mpi).
• [ ] The AWS system doesn't need to define a new scheduler; should the scheduler code be removed from it?
• [ ] The AWS system needs a dry run (should be addressed in response to #455)
• [ ] Derived class still specifies the scheduler on the system, which will be used by default (when the scheduler is not specified during system init).
• [ ] Is there a way to clean up the code in the system-specific system.py? values=("flux", "mpi") with "flux" being the default option seems brittle.
• [ ] Is there a way to make the scheduler a binary variant? i.e., scheduler=True uses flux, scheduler=False uses mpi.

[scheibelp]

Now that Tioga has a new variant, the hash location of the Ramble workspace has changed (sorry - https://github.com/LLNL/benchpark/pull/450 ought to help with this)