CrushWrapper: pick a ruleset same as rule_id

Originally in the add_simple_ruleset funtion, the ruleset_id
is not reused but rule_id is reused. So after some add/remove
against rules, the newly created rule likely to have
ruleset!=rule_id.

We dont want this happen because we are trying to hold the constraint
that ruleset == rule_id.

This patch allow ruleset_id be reused. After having this patch,
add/remove rules will not break the constraint.

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>
I'm not sure this comment will be helpful in the future. It is clear what it means right now though ;-)