osd: Add config options for cost per io & byte for the mclock scheduler

osd: Add config options for cost per io & byte for the mclock scheduler

The cost per io and cost per byte options for hdd and ssd are specified
and set to default values determined using experiments on hdds and ssds
using a cost model. The values are used in calc_scaled_cost() to
determine the scaled cost for every OpSchedulerItem that is enqueued
within the mClockScheduler.

Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>
(cherry picked from commit 2da091229bd3a9c4d81fecacb60b918a614aeb84)