Fix the qcow2 disk image content

Fix the qcow2 disk image content

When building the qcow2 image, we are not automatically getting the
latest version of the samplevnf code. Since this might have to do with
caches, a parameter with the workspace directory is now passed to
disk-image-create using the --image-cache option.
Also removed verify-image.sh since this is done using build-image.sh
with the -v option.

Change-Id: I661c543102258929bdfe53af6c6ea0ff72eab33d
Signed-off-by: Luc Provoost <luc.provoost@intel.com>