cloud-images: Change the Atomic Vagrant box size to 40
    
    This matches the current Base Vagrant.  Having large disk images (that
    should be thinly provisioned on the host, by e.g. qcow2) is a normal
    user experience for Vagrant.
    
    See: https://lists.projectatomic.io/projectatomic-archives/atomic-devel/2015-May/msg00046.html
    
    Signed-off-by: Colin Walters <walters@verbum.org>