buildinstall: Expose lorax's --rootfs-size argument
    
    We already make this possible for the ostree installer, but it was
    missing from the traditional one. The default behaviour is to let lorax
    decide, but if user knows better, they can overwrite in configuration.
    
    JIRA: COMPOSE-3188
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>