Figure out spot instance nuances

[kurtwheeler]

Context

Spot instances cost a fraction of the normal price for AWS instances. The downside is that they can be preempted if the demand for AWS instances becomes too high. However this is not a big deal for our system because we don't need to have data processed ASAP, it's worth the delay to save money.

Problem or idea

We should use spot instances instead of normal AWS instances.

Solution or next step

Switch the instances used for Nomad clients to spot instances. The best way to do this will probably require a bit of research to determine, but there are some blog posts and there is also a specific section in the terraform docs about it.

The one thing that is known is that we definitely should use the auto-scaling group created in #61.