add support for slurm

[o-smirnov]

E.g. ilifu only allows containers to be run via the scheduler (sbatch, srun, sinteractive). The latter option probably makes the most sense, since it takes care of killing the session on disconnect. Now need to:

• provide instructions for manually building the image (or doing that via srun)

• collect port forwarding info from output. How to select a port if we don't know the node?

[o-smirnov]

Done in 1.2.pre5.