cloud-atomic: Re-shrink rootfs partitions
    
    mattdm changed these previously because we hit an ENOSPC error during
    qcow2 image creation, but that *really* happened because the Fedora
    rel-eng create-cloud-images script hardcoded 3GB.
    
    Now that we've shrunk the content down to ~850MB, drop the defaults:
    
    - rootfs down to 2GB.  This is enough space for a replacement of
      all content, plus a bit of spare room for logging
    - /boot to 200MB.  At present, kernel+initramfs is just 30MB,
      so this is still very conservative.