Add support for AWS Spot Instances/other options besides On-Demand instances.

[jgreenemi]

The tool is set to launch servers in the On-Demand way, meaning this is the most expensive way to go about it. This should be changed such that users can introduce the use of Spot instances to save on costs, or even take advantage of Reserved Instances if they have RI capacity already set aside. I'm unfamiliar how the latter works when used with CFN stacks, so this will require additional research.