Support Volume Num Weighter

Currently cinder support choosing volume backend according to
free_capacity and allocated_capacity. Volume Num Weighter is
that scheduler could choose volume backend based on volume number
in volume backend, which could provide another mean to help
improve volume-backends' IO balance and volumes' IO performance,
see details in ref [1] and [2].

blueprint support-volume-num-weighter

Ref:
[1]https://blueprints.launchpad.net/cinder/+spec/support-volume-num-weighter
[2]https://review.openstack.org/#/c/99683/

Change-Id: Id9275ed954f1b35ee8d7bee0f0b61cc3a7c48f63
Merge "Support Volume Num Weighter"