Make the parameter 'device' optional

As http://developer.openstack.org/api-ref/compute/?expanded=attach-a-volume-to-an-instance-detail
the parameter 'device' is optional and Nova Liberty+ clouds do
auto-assignment as the device with/without specifying the parameter.
This patch makes the parameter optional on AttachVolumeTestJSON.

Change-Id: I78c721c7c1c27460825ce483304d81864a39379b
1 file changed