Add the ability to separate architecture in path scheme (#4329)

* Add format to separate target and os for path

spec format can now handle separations of target and os for setting
up the path.

* Added ${PLATFORM} et al to spec.format()

${PLATFORM}, ${OS}, ${TARGET}

* Update tests

Updated tests and got rid of unnecessary code.

* Also update documentation to reflect this new ability.

* Add default path scheme to config.yaml

Added default path scheme to config.yaml. Users can overwrite this
section if they want.

etc/spack/defaults/config.yaml

@@ -22,6 +22,9 @@ config:
   template_dirs:
     - $spack/templates
 
+  # default directory layout
+  directory_layout: "${ARCHITECTURE}/${COMPILERNAME}-${COMPILERVER}/${PACKAGE}-${VERSION}-${HASH}"
+
   # Locations where different types of modules should be installed.
   module_roots:
     tcl:    $spack/share/spack/modules