Extension of storage reporting design doc

This patch rewrites and extends the design doc about storage reporting
with respect to disk templates and storage types. In constrast to the
previous version, we now consider disk templates as the user-facing
entity, that the user can dis/enable for the cluster. Storage types
on the other hand describe the underlying technology used by the various
disk templates. Storage reporting will use a mapping from disk templates
to storage types to pick the correct method to report the storage for
the respective disk templates.

Note that the design doc in this state still contains some questions and
FIXMEs. Feel free to comment on those. I will complete them directly or
in future patches.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>