Fix to enable ec2-fetch-ssh-public-key init script

The init script was not running on boot because the init script
symlinks were not created in /etc/rcX.d for the runlevels: 2,3,4,5.

This fix runs the update-rc.d command in the is_amazon_ec2_ami
function, thus creating the required symlinks and also updating
/etc/init.d/.depends.on.start.