Request spot instances

[seankelly]

The max price is set to $0.05. In us-west-1, one region consistently has a price of around $0.032 for m3.xlarge.

I've never used EC2 instances but this appears to be all that is necessary to request spot instances.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[brson]

Wow, thanks for this patch! This is definitely something we want to pursue.

Unfortunately, I've tried exactly this before and there's more to it. The first error we're going to see is that the AWS account buildbot uses is going to need to have new permissions added. The one time I tried this I could not get it to work correctly.

It's going to require a lot of validation in the dev environment too to have confidence that spot instances will work reliably.

cc @edunham

[brson]

Closing since this is old and will take a lot of consideration to pull off.