Support creating users on login

[zeffron]

aws ec2-instance-connect send-ssh-public-key (in the AWS CLI) lets me upload an SSH for users that do not exist. This makes sense and is good.
I would like to be able to have those users created on-login, though (according to a provided user template, perhaps).
Our use case is to allow developers to come and go from the team without needing to manually create accounts for them on our fleet of hundreds of instances to grant them the ability to login to those instances as required.

[myoung34]

I did a sort of dirty first pass to look into this here