Merge pull request #1309 from LLNL/features/spack-defaults

Add "default" configuration scope.

lib/spack/spack/config.py

@@ -497,8 +497,15 @@ def clear(self):
         """Empty cached config information."""
         self.sections = {}
 
+"""Default configuration scope is the lowest-level scope. These are
+   versioned with Spack and can be overridden by sites or users."""
+ConfigScope('defaults', os.path.join(spack.etc_path, 'spack', 'defaults'))
 
-ConfigScope('site', os.path.join(spack.etc_path, 'spack')),
+"""Site configuration is per spack instance, for sites or projects.
+   No site-level configs should be checked into spack by default."""
+ConfigScope('site', os.path.join(spack.etc_path, 'spack'))
+
+"""User configuration can override both spack defaults and site config."""
 ConfigScope('user', os.path.expanduser('~/.spack'))