Add "default" configuration scope.

- Default scope is versioned with spack and can be overridden by site
  or user config.

- Default scope provides sensible default concretization preferences
  for all of Spack.

- per-platform concretization scope can be added later (to force a
  particular MPI on, e.g., Cray systems)

.gitignore

@@ -5,7 +5,7 @@
 *~
 .DS_Store
 .idea
-/etc/spack/*
+/etc/spack/*.yaml
 /etc/spackconfig
 /share/spack/dotkit
 /share/spack/modules