When attaching a volume to an instance, return the chosen device name to the app.

There is an application need for Murano to return the device name that the instance assigns to a volume when the attach volume
request is made. An application developer can assign a device name to a volume under the block devices property, deviceName, but this name may or may not get used when the attach is done. Therefore this property cannot be relied upon to provide the actual device name that the volume is assigned to. One possible solution is to put the assigned device name into a new property of the entries,"blockDevices" and "volumes", upon deployment.