Add support to Capacity Based QoS to Ceph RBD

Add a support for a Ceph backend Capacity QoS using the following keys
total_iops_sec_per_gb
read_iops_sec_per_gb
write_iops_sec_per_gb
read_bytes_sec_per_gb
write_bytes_sec_per_gb
total_bytes_sec_per_gb

Cinder can perform the calculation based on volume size and set the QoS Value directly on ceph image

Example:

total_iops_sec_per_gb = 3
100GB volume

when a volume is created/resized/migrated will set the qos property
100 * 3 = 300

so the rbd_qos_iops_limit will be set on =300