Fix AWS EC2 AMI support

- grub.cfg is now correctly modified when installing on a VyOS AMI
- ec2-fetch-ssh-public-key init script still needs to be activated
  if installing on a VyOS AMI. This is an outstanding issue which
  needs to be addressed. See debian/vyatta-cfg-system.postinst.in.
0.20.43+vyos1+hydrogen5