Reuse current spindles in design for partitioned

Renaming the current parameters dealing with spindles is nice, as their
meaning would be clearer, but that would mean changing a lot of code,
change external interfaces, and renaming internal variables for little
benefit. Also, instance policies would end up containing two related
parameters, one used when exclusive_storage is enabled, and the other when
disabled. Reusing the existing parameters by changing slightly their
semantics makes more sense.

Signed-off-by: Bernardo Dal Seno <bdalseno@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>