Update gpfs volume driver path

* Change of driver path from "drivers.gpfs.GPFSDriver" to
  "drivers.ibm.gpfs.GPFSDriver"
* Fix issue of assigning "gpfs_images_share_mode=copy_on_write"
  even if the parent attribute "gpfs_images_dir" is not defined

Change-Id: Ica00f00901502857d5d21ef7ce7ae3b68e4ca9b7
Closes-Bug: #1389955