configure-time view variable templating

[bcumming]

Configure time string substitution of mount and view information for environment variables defined in the env:views:view:env_vars field.

Substition of a key is performed for templates of the form $@key@, e.g.:

cuda-env:
  views:
      env_vars:
          set:
          - ACTIVATE: "$@mount@/activate.sh"

The following keys are supported, with examples:

mount.            /user-environment
view_name     cuda-env
view_path       /user-environment/env/cuda-env