vzdump: apt: notification: do not include domain in 'hostname' field

vzdump: apt: notification: do not include domain in 'hostname' field

 - The man page warns about the usage of `hostname -f`, since a host
   may have multiple domains (or none at all)
 - The fallback PVE::INotify::nodename() already only returned the
   hostname without the domain part
 - Fencing notifications didn't include the domain part anyway

This may result in soft-breakage for any users who have already relied
on the domain being present. If there is need for it, it could include
a fqdn metadata field.

The hostname property used for rendering the notification template
is unaffected for now.

Signed-off-by: Lukas Wagner <l.wagner@proxmox.com>