cloudinit: make genisoimage only output errors

cloudinit: make genisoimage only output errors

avoids a genisoimage output like:
> Total translation table size: 0
> Total rockridge attributes bytes: 417
> Total directory bytes: 0
> Path table size(bytes): 10
> Max brk space used 0
> 178 extents written (0 MB)

on every VM start.

Rather than that useless output, tell genisoimage to be quiet, which
still prints errors but nothing else. Additionally print a short
single line about that we're to create the cloud-init iso.

Reformat while at it.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>