Test dynamic AWS system

[pearce8]

@wdhawkins The AWS system definition has been updated (systems/aws-pcluster), please test and let us know if you think it does what it should. Right now, this produces experiment scripts like:

#SBATCH --mpi=pmix --export=ALL,FI_EFA_USE_DEVICE_RDMA=1,FI_PROVIDER="efa",OMPI_MCA_mtl_base_verbose=100
#SBATCH -n 8
#SBATCH -N 1
...
srun --mpi=pmix --export=ALL,FI_EFA_USE_DEVICE_RDMA=1,FI_PROVIDER="efa",OMPI_MCA_mtl_base_verbose=100 -n 8 -N 1 saxpy -n 512 >> ...

(i.e. these are generated as sbatch opts and command opts) - this is decided by the allocation modifier (specifically for the SLURM scheduler). I think it's worth checking if these should be batch opts

• If so, this can be rewritten to generate a separate batch opt for --mpi and --export
• If not, I can change the allocation modifier

I don't think this decision need stop this PR, but the generated experiment scripts will have weird sbatch directives until that is decided.