Default to scope with highest precedence instead of user scope,

- Generalizes config scopes a bit more: nothing assumes there is a
  'user' scope (this would break testing sometimes).

lib/spack/spack/compilers/__init__.py

@@ -84,7 +84,7 @@ def get_compiler_config(arch=None):
         compilers = find_compilers(*get_path('PATH'))
         for compiler in compilers:
             config[arch].update(_to_dict(compiler))
-        spack.config.update_config('compilers', config, 'user')
+        spack.config.update_config('compilers', config)
 
     # Merge 'all' compilers with arch-specific ones.
     merged_config = config.get('all', {})

lib/spack/spack/config.py

@@ -193,8 +193,16 @@ def clear(self):
 
 
 def check_scope(scope):
+    """Ensure that scope is valid, and return a valid scope if it is None.
+
+       This should be used by routines in ``config.py`` to validate
+       scope name arguments, and to determine a default scope where no
+       scope is specified.
+
+    """
     if scope is None:
-        return 'user'
+        # default to the scope with highest precedence.
+        return config_scopes[-1]
     elif scope not in valid_scopes:
         raise ValueError("Invalid config scope: '%s'.  Must be one of %s."
                          % (scope, valid_scopes))