Amazon Linux additional SSH users
- https://blog.e-zest.com/how-to-add-ssh-users-in-amazon-linux/
- https://stackoverflow.com/questions/24409095/create-an-ec2-instance-with-multiple-key-pairs
- http://www.beginninglinux.com/home/server-administration/openssh-keys-certificates-authentication-pem-pub-crt