Add experimental gce-disk-expand package for EL6.
Add experimental gce-disk-expand package for EL6.

illfelder/compute-image-packages,Sarsate/compute-image-packages,zmarano/compute-image-packages,wrigri/compute-image-packages,wrigri/compute-image-packages,rjschwei/compute-image-packages,rjschwei/compute-image-packages,Sarsate/compute-image-packages,wrigri/compute-image-packages,GoogleCloudPlatform/compute-image-packages,wrigri/compute-image-packages,illfelder/compute-image-packages,zmarano/compute-image-packages