Implement front end rate-limiting for Cinder volume

Cinder adds Quality-of-Service control interface to allow admin to
specify QoS parameters as well as the place where these parameters
should be consumed (Front-end: Nova Compute, Back-end: Cinder
Volume, or Both) for volume types.  KVM and other hypervisor is
known to be able to do disk rate-limit control (one form of QoS)
for virtual disks, this patch enables Nova Compute to extract
rate-limit parameters for volumes while attaching, currently
libvirt only.

implement bp volume-rate-limiting

Change-Id: Ibcc8f15ab744600e6f814b46935f8022d78074fc
Merge "Implement front end rate-limiting for Cinder volume"