add an option to ch-run to suppress forwarding of SLURM env variables to processes in the container

Created by: hppritcha

Hunter and I are going around in circles on some edge cases with Open MPI and doing

• singleton launch within a container (that means running a MPI executable without using mpirun, etc.)
• mpirun launch within a container which was in turn started within a SLURM allocation

These problems would vanish if there were a ch-run command line option to suppress forwarding of SLURM environment variables to the processes (including mpirun) within the container.