support non-terraform means of specifying hosts

[rustyrazorblade]

Terraform is great at provisioning and can supply all the hosts, but by tightly coupling the install to Terraform's database we make it impossible to set up labs on non-AWS environments. It also assumes the individual running the tests has the ability to create servers, which might be restricted. Security sometimes restricts what people can do, and it's possible someone might have to ask for servers to be provisioned and only get a block of IPs.

As a catch all, it would be nice to be able to specify a hostfile, something like this: --hostfile my-hosts.yaml

The hostfile could contain sections for cassandra, stress and monitoring. Then we have the ability to use tlp-cluster on non-aws environments. I don't think we need anything other than the public IPs for this but this should be verified.