Facilitate login to Stark & Whitewalkers from the Nightking node

One nice feature to have would be to allow (perhaps while in DEV mode only) access to the public ips of all stark and whitewalker nodes. This would primarily be useful to ease the amount of work necessary for ssh-ing into the virtual machines of these nodes for manual inspection.

I can imagine capturing the public ip addresses of stark/whitewalker nodes in environment variables such as `$STARK1_PUBLIC_IP`, for instance implemented in the `library.bash` script on the Nightking node. Alternatively, a utility function could be provided, say `ssh-experiment-node` that would take as parameter the name of the target node, say `--id "stark1"`. Behind the curtains, this utility could simply unwrap into `ssh -l ec2-user -i /var/log/nightking/pool.key _public_ip_stark1`.