Configure and use volume number weigher

Cinder volume number weigher(https://review.openstack.org/#/c/102414/) has
been merged into cinder.
It needs add docs for how to configure and use volume number weigher.

Change-Id: I1dcf2424b8ce7cf349c88103484901fe3db5d16f
Closes-Bug: #1338456
Merge "Configure and use volume number weigher"