Fix Greeter Login panel on Ubuntu 17.04

This works with all flavors of Linux that use LightDM as their greeter.

When my laptop was in its docking station, the login prompt would not display.  I guess it was displaying on the closed laptop display.

Create a file /opt/sbin/set-prime-mon

Contents:

#!/bin/sh
# Set prime monitor to left most
LOG=/var/log/set-prime-mon.log
# Remove any previous logs
rm -f $LOG
output=$(xrandr | grep -E " connected (primary )?[1-9]+" | grep "+0+0" | sed -e "s/\([A-Z0-9]\+\) connected.*/\1/")
echo $output >> $LOG 2>&1
if [ -n $output ]; then
 echo "setting prime to $output" >> $LOG 2>&1
 xrandr --output $output --primary
fi

Create a file /etc/lightdm/lightdm.conf.d/99-setprime.sh

Contents:

display-setup-script=/opt/sbin/set-prime-mon