AWS: autogenerate VPC and subnet

[cgoettel]

The VPC and Subnet are currently hardcoded. They should be generated automatically.

Historical note: I worked on this extensively in #8, but the instance was never accessible via Systems Manager (SSM) and port 4000 never opened (so something may have been wrong with the install? Hard to tell without being able to log into the instance). That effort was not part of #8 so I scratched it and moved on, documenting it here instead. Happy hunting. Further note: I'm willing to accept that this whole issue may just be an artifact of the weird way I have to authenticate into AWS. Others may not hit this. If that's the case, let's go nuts with the commented out code in aws/network.tf.