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)

etc/spack/defaults/packages.yaml

@@ -0,0 +1,21 @@
+# -------------------------------------------------------------------------
+# This file controls default concretization preferences for Spack.
+#
+# 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/packages.yaml
+#
+# Per-user settings (overrides default and site settings):
+#   ~/.spack/packages.yaml
+# -------------------------------------------------------------------------
+packages:
+  all:
+    providers:
+      mpi: [openmpi, mpich]
+      blas: [openblas]
+      lapack: [openblas]