Request Spot Instances from AWS EC2

[kiwialec]

This change requests spot instances instead of the default on-demand ones.

I know this project is dead, but I took some code from other PRs. Hopefully this one saves someone some time.

[nichoi]

Hi, @kiwialec , seeing as you're using EC2 as your provider, wondering if you encountered this issue?

https://github.com/fabienvauchelles/scrapoxy/issues/189

[fabienvauchelles]

good candidate for 4.0.0