Booting into text mode in 16.04
To boot Ubuntu 16.04 Desktop without X one time, add systemd.unit=multi-user.target to the linux command line in GRUB.
To make this the default, use
sudo systemctl set-default multi-user.target
To return to default booting into X, use
sudo systemctl set-default graphical.target
To see the current default target,
sudo systemctl get-default