Attempt to retrieve the vmdk descriptor data-pair

VMDK formats such as monolithicFlat and vmfs require two files to be
fully consumable by the Nova drivers (a descriptor-data pair: *.vmdk and
*-flat.vmdk).
On the upload of the descriptor (*.vmdk), upload_image.sh should attempt to
retrieve the *-flat.vmdk. The same way, the descriptor should be
retrieved when a flat disk is uploaded.
On success, the upload script will be able to use the flat disk as the image
content and the relevant descriptor settings as the image metadata.

Change-Id: I9214754029c46dd60b9e7d606d84d8819a498a8d
Closes-Bug: #1252443
Merge "Attempt to retrieve the vmdk descriptor data-pair"