nova should provide api for getting disk usage of compute node

Currently there is no way for admin users to have a clear knowledge of how much disk capacity left totally on the compute node, and disk capacity is needed for vm deployment and other temporary use, for example image-downloading, instance snapshoting, and image conversion and so on. If there is little disk space left on the host, all those operations would failed at last, which would confuse admin user. There needs to be a way for Horizon to discover the disk capacity and usage on each compute node, api for getting disk capacity and usage of compute node would be helpful.