htools: add spindle-related attribute to nodes

This adds the spindleCount, hiSpindles and instSpindles attributes.

The spindleCount is equivalent to spindle_count on the ganeti side (a
node parameter). hiSpindles is the maximum instance-used spindles, and
instSpindles will be used to track spindles as used by instances (in
later patches).

The patch also reads spindleCount from the live backends. The text and
simu backends currently set it to 1, hard-coded.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>