add support for spot instances

I extracted the code related to spot instances support from [curiousjgeorge fork](https://github.com/curiousjgeorge/jmeter-ec2/commit/58833103b0c36a69d51d017d15fcbd63452c8ea2)

I also added error handling in case all spot requests failed.

Tell me if you're interested, i can add some notes to the readme file