Rename install.yaml -> config.yaml, install_area -> "store"

- Added a schema for config.yaml
- Moved install tree configuration to config.yaml
- Moved etc/spack/install.yaml to etc/spack/defaults/config.yaml
- renamed install_area to "store", to use a term in common with guix/nix.
  - in `config.yaml` file, it's called the `install_tree` to be more
    intuitive to users.
  - `install_tree` might've worked in the code, but `install_tree` is
    already a global function in the spack namespace, from
    llnl.util.filesystem.