The attach workflow is as following:
* reserve_volume in Cinder
* initialize_connection in Cinder
* connect_volume in os_brick
* attach in Cinder
If the workflow executes sucessfully, the Cinder volume
is attached to the compute host as a device. Zun will
mount the device to the filesystem in order to use it.
The detach workflow is as following:
* begin_detach in Cinder
* disconnect_volume in os_brick
* terminate_connection in Cinder
* detach in Cinder
Partial-Implements: blueprint direct-cinder-integration
Change-Id: Ib47e76d14b3dc036f903a3988bb138288cdb601e
18b3b059c4