"mountpoint" should not be required parameter

The nova server supports attaching a volume to a server without taking
device name, so the "mountpoint" should not be required parameter on
VolumeAttachment in heat.

Change-Id: If5c978345c916a319cc150fe7edd86d5056c5b1f
Closes-Bug: #1265419
Merge ""mountpoint" should not be required parameter"