Add support for instance disk IO control.

Part 3 for blueprint quota-instance-resource, by setting exposed params
"disk_read_bytes_sec, disk_read_iops_sec, disk_write_bytes_sec,
disk_write_iops_sec, disk_total_bytes_sec, disk_total_iops_sec"
through nova flavor-set, user can implement instance disk IO control
policy.
for example:
nova flavor-set m1.medium set disk_write_bytes_sec=10240000,
then the max disk write IO is 10MB/s for VM user.

partly implement blueprint quota-instance-resource
original reviews https://review.openstack.org/#/c/14151/
DocImpact

Change-Id: I8605acf61d7ddbfdaf6eee3ab135898df2449484
Merge "Add support for instance disk IO control."