Allow runtime_path to be set using variables in plugstack.conf

[lambda-brian]

It would be useful if we could use environment and SLURM variables in the plugstack.conf file.

In particular, I would like to be able to do something like:

required /usr/lib/x86_64-linux-gnu/slurm/spank_pyxis.so runtime_path=/data/${SLURM_JOB_USER}/pyxis
Currently, trying to do that passes the literal '${SLURM_JOB_USER}' which gets created.

[flx42]

Will the need for this be alleviated by using the new enroot feature enroot load? NVIDIA/enroot@0b92ce3

If enroot load is used (when new releases are done), there will be no need for an intermediate squashfs file.

[lambda-brian]

That may well do the trick. It will likely need to get implemented in the plugin once available, but once done, it should even speed things up nicely.