This rather lenghy patch comprises a couple of refactorings
to achieve the following goals:
- 'gnt-node info' should only report space information about
  spindles, when exclusive storage is enabled on the node
  (so far it always did when lvm-based storage was enabled).
- 'gnt-node info' should report the disks' space information
  for the default disk template's storage. So far it did it
  always for LVM, which is irrelevant for a cluster running
  for example file-based instances.
- Performance tweaking of storage reporting and gnt-node
  info.
- Consider the future work on storage pools for any changes.

This is is achieved by the following refactorings:
- Move the decision whether or not to ask for spindle
  from cmdlib-level to rpc level.
- Move the decision for which disk template's storage
  information to ask for to cmdlib level (instead of
  requesting information for all storage units and
  later discarding most of them).
- Adjustments to storage utility functions.
- Affected unit tests are adjusted as well.

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