Ability to use env variables in settings.yaml values

[marcodelapierre]

It would be great if this feature were available, in particular for registry, module_base and container_base.

Current behaviour is as follows.

Snippet from settings.yaml:

module_base: $MYGROUP/devel/install-spack-shpc-test-variables/zio-shpc/modules

container_base: $MYGROUP/devel/install-spack-shpc-test-variables/zio-shpc/containers

Installation succeeds, but a directory called $MYGROUP is created:

$ ls

bin           Dockerfile      docs           env.sh                lib      MANIFEST.in  $MYGROUP  README.md  setup.py  singularity_hpc.egg-info
CHANGELOG.md  Dockerfile.tcl  entrypoint.sh  install-shpc-zeus.sh  LICENSE  modules      paper     registry   shpc

[vsoch]

This should be do-able if we just do the replacements, and then run an expandvars command.