This patch improves the handling of storage information
before and after a call to the RPC 'node_info'. It
adds a function to not only call for all storage
information on the cluster (as it is used right now),
but to ask only for storage information for a particular
disk template. This way, in many cases, for example
by the iallocator, less information is requested and thus
performance is increased. Unit tests are provided.

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