Remove Costs from the 'Weights and Costs' Chapter

Costs are no longer used in the scheduler, so Weights chapter is
updated with the remaining ram_weight_multiplier option.

Fixes bug 1172264

Cherry-picked from https://review.openstack.org/#/c/30970

Change-Id: Ic8c57b2107c1e334acb1adfe3498d33fde7b435d
Merge "Remove Costs from the 'Weights and Costs' Chapter" into stable/grizzly