Allow the Provisioning of a Public SSH Key From GitHub to the Bastion

[Sam Irvine]

Proposed approach
When we run perturb or simulator scenario launch we could take an option for a list of GitHub usernames. Github allows anyone to view the public SSH keys added to an account at the https://api.github.com/users/:user/keys endpoint. On the bastion this can be used to provision the public SSH keys from these accounts as acceptable keys for the ubuntu user. Multiple people would then be allowed to SSH to the bastion at once with only the knowledge of the bastion IP. So it would make it a lot easier to share a simulator session and allow people to connect without distributing private SSH keys.

This feature would probably need to be used in tandem with the --extra-cidrs option on simulator infra create to get the full effect.