Track instance-used spindles in node set/add/remove

This simply tracks the instance-used spindles (using hard-coded '1'
per instance), and additionally prevents additions in soft-mode when
we go over the limit.

Note: there's an assymetry between addSec and removeSec (basically
secondaries should only exist with DTDrbd8, so the check for uses_disk
is superflous), will be cleaned up later.

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