Rename design doc from space reporting to storage types

Since the design doc for storage space reporting ended up being mostly
about disk templates and storage types, and the space reporting being
just a small part of it, I am renaming the design doc.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

doc/design-storagespace.rst → doc/design-storagetypes.rst

-============================
-Storage free space reporting
-============================
+=============================================================================
+Management of storage types and disk templates, incl. storage space reporting
+=============================================================================
 
 .. contents:: :depth: 4
 
@@ -12,9 +12,6 @@ in Ganeti. One direct consequence is that storage space reporting is currently
 broken for all storage that is not based on lvm technolgy. This design looks at
 the root causes and proposes a way to fix it.
 
-FIXME: rename the design doc to make clear that space reporting is not the only
-thing covered here?
-
 Proposed changes
 ================