Implement basic feature to mount modular uenvs from the json description.

[simonpintarelli]

• mount modular uenv from json description: https://github.com/eth-cscs/stackinator/blob/feat/modular-sqfs/stackinator/schema/base-uenv.json
• an image is described as

         "image": {
           "name": "osu-micro-benchmarks@daint",
           "prefix_path": "/user-environment/",
           "file": "/capstor/scratch/.../osu-daint.sqfs",
           "sha256": "c4e990bcbdf261ab627143596a811ca239637d861b3e6b3e5b69f421dc3830d1"
       }

Currently only file and prefix_path are used. Idea is to use sha256 once the base images are in the uenv repo.

• TODO: if the base uenvs contains the meta directory, uenv status will discover the views in the base uenvs (since they appear in UENV_MOUNT_LIST). Either remove them from UENV_MOUNT_LIST or build the base uenvs without the env.json.