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.

etc/spack/defaults/config.yaml

@@ -0,0 +1,64 @@
+# -------------------------------------------------------------------------
+# This is the default spack configuration file.
+#
+# Settings here are versioned with Spack and are intended to provide
+# sensible defaults out of the box. Spack maintainers should edit this
+# file to keep it current.
+#
+# Users can override these settings by editing the following files.
+#
+# Per-spack-instance settings (overrides defaults):
+#   $SPACK_ROOT/etc/spack/config.yaml
+#
+# Per-user settings (overrides default and site settings):
+#   ~/.spack/config.yaml
+# -------------------------------------------------------------------------
+config:
+
+  # Precedence of configuration scopes, high to low.
+  # The user can override this paradoxically.
+  # scopes: [user, spack, default]
+
+
+  # This is the path to the root of the Spack install tree.
+  # You can use $spack here to refer to the root of the spack instance.
+  install_tree: $spack/opt/spack
+
+
+  # Temporary locations Spack can try to use for builds.
+  #
+  # Spack will use the first one it finds that exists and is writable.
+  # You can use $tempdir to refer to the system default temp directory
+  # (as returned by tempfile.gettempdir()).
+  #
+  # A value of $local indicates that Spack should run builds directly
+  # inside its install directory without staging them in temporary space.
+  build_stage:
+    - /usr/workspace/*/%u
+    - $tempdir
+    - /nfs/tmp2/%u
+    - $local
+
+
+  # Cache directory already downloaded source tarballs and archived
+  # repositories. This can be purged with spack purge
+  source_cache: $spack/var/spack/cache
+
+
+  # Cache directory for miscellaneous files, like the package index.
+  misc_cache: ~/.spack/cache
+
+
+  # If this is false, tools like curl that use SSL will not verify
+  # certifiates. (e.g., curl will use use the -k option)
+  verify_ssl: true
+
+
+  # If set to true, Spack will always check checksums after downloading
+  # archives. If false, Spack skips the checksum step.
+  checksum: true
+
+
+  # If set to true, `spack install` and friends will NOT clean
+  # potentially harmful variables from the build environment. Use wisely.
+  dirty: false