Make Workstation disk image size "16GB" (RHBZ #2247611)
    
    This is 16 power-of-ten gigabytes, the type of gigabyte generally
    used to specify real physical media capacity. '16' gives us 16
    power-of-two gigabytes, which is bigger than a typical 16GB USB
    stick.
    
    All the oz/ImageFactory and Koji changes needed to make this
    possible should be merged and in production now, I think.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>