Bugfix: '::' only worked on top-level key in config.

- generalized and fixed to work with any key in YAML file
- simplified schema writing, as well
- add more unit tests for the config system
- Rename test/yaml.py to test/spack_yaml.py
  - Add test/yaml.pyc to ignored pyc files.

bin/spack

@@ -72,7 +72,8 @@ orphaned_pyc_files = [
     os.path.join(SPACK_LIB_PATH, 'spack', 'platforms', 'cray_xc.pyc'),
     os.path.join(SPACK_LIB_PATH, 'spack', 'cmd', 'package-list.pyc'),
     os.path.join(SPACK_LIB_PATH, 'spack', 'cmd', 'test-install.pyc'),
-    os.path.join(SPACK_LIB_PATH, 'spack', 'cmd', 'url-parse.pyc')
+    os.path.join(SPACK_LIB_PATH, 'spack', 'cmd', 'url-parse.pyc'),
+    os.path.join(SPACK_LIB_PATH, 'spack', 'test', 'yaml.pyc')
 ]
 
 for pyc_file in orphaned_pyc_files: