JeOS 19 ks: Fill up disk image so empty space can be removed

Add a last step post install that will make the JeOS
image to fill all 10GB of disk, so that we can easily
create an image with no empty space by using qemu-img

qemu-img convert -O qcow2 jeos-19-x86_64.qcow2 jeos-19-x86_64-output.qcow2

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1225 from autotest/jeos-19-fillup

JeOS 19 ks: Fill up disk image so empty space can be removed