VPC Support

[jonhoo]

Amazon EC2 has a notion of a Virtual Private Cloud (see also the VPC intro). tsunami currently just spawns all instances in the default VPC, and uses security groups to ensure that the resulting instances can only talk to each other. Instead, we should instead set up a new VPC for our job, spawn the instances inside of it, and set up the firewall such that the only traffic allow in is SSH.

Implementing this will resemble what we did for security groups; specifically, first create a VPC with some semi-random name, and then use the resulting VPC ID when launching the spot requests. Note that the VPC IP range will also need to be included in the security group setup, and that the VPC should be torn down after the job has finished.

[bsodmike]

@jonhoo please see the PR when time permits. I realise that many use Terraform now, so I'm not sure if this crate is still as useful (?), that said, it's a nice idea to have this for anyone looking to manage AWS resources with Rust.