Make name and description nullable for virtual disks
Make name and description nullable for virtual disks

This change fixes inspection failure on my hardware.
In the future we should develop a better understanding on which
fields are mandatory and which are not, as right now we're very
inconsistent with handling them.

Change-Id: Ia2af90fec4eef168953552a947933617e7c808d0
Closes-Bug: #1550328

openstack/python-dracclient