slurm: Avoid fixed paths for Slurm, and avoid shell init

Our Slurm install doesn't live in /usr/bin, so we need to drop
the full path for these commands.

Also the -i option to the shell triggers some things that are
only done on login (like printing CPU quota) so we drop that too.
Merge pull request #1 from chrissamuel/master

slurm: Avoid fixed paths for Slurm, and avoid shell init