The display name of volume which synchronized form PowerVC is None

To enhance the code to fetch volume display_name for latest PowerVC
If fetched "display_name" is empty then fetch "name" instead, if
still empty then just display local os as empty as previous.

Change-Id: I43161b50d153d9ed3750f8a2ea740c0dce633e35
Closes-Bug: #1363846