cloudinit: pass through hostname via fqdn field

cloudinit: pass through hostname via fqdn field

If no FQDN is provided, we simply set it to the current hostname. This
ensures that the hostname *really* gets set, since we encountered an
issue on Fedora and CentOS based systems where no hostname got set at
all.

When there's no FQDN set in the cloudinit config, this leads to the
following entry:

    127.0.1.1 <hostname> <hostname>

Which doesn't seem to cause any issues.

Tested on:
 - Ubuntu 23.04
 - CentOS 8
 - Fedora 38
 - Debian 11
 - SUSE 15.4

Signed-off-by: Leo Nunner <l.nunner@proxmox.com>