[ONE images] do not use hostnamectl in chroot (ubuntu build script)

opennebula-images/ubuntu-build-opennebula-image.sh

@@ -140,7 +140,7 @@ EOF
 run_root rm -f /etc/machine-id
 run_root touch /etc/machine-id
 rm -f /var/lib/systemd/random-seed
-run_root hostnamectl set-hostname ubuntu
+echo "ubuntu" > /mnt/etc/hostname
 
 # Remove temporary files and reclaim freed disk space.
 run_root apt-get clean