Add QoS support to IBM Storwize driver

IBM Storwize driver can be enabled with QoS support by setting the parameter
I/O throttling rate, which caps the amount of I/O.

This patch add the QoS configuration to the methods of create_volume,
create_volume_from_snapshot, create_cloned_volume and retype.

The QoS for IBM Storwize storage can be configured in 3 ways:
* Add the key "qos:IOThrottling" into a QoS spec and associate this QoS
spec with a volume type.
* Add the key "qos:IOThrottling" into the extra spec of a volume type.
* Add a metadata with the key "qos:IOThrottling".

Change-Id: I7c5c6726a61efad6a8ada9c5dcc31a9382ec79f9
implements-blueprint: cinder-storwize-driver-qos