"Rate exceeded" with default retry limit

[amcinnes]

My team has some projects which involve deploying about 10 stacks in parallel to the same AWS account and region. If we deploy multiple of these projects at once we see stackup failing with "Rate exceeded" error.

Our developers and/or per-application deployment scripts ideally shouldn't have to think about setting a specific --retry-limit option when they deploy, as this is a generic CloudFormation API concern rather than an application-specific concern. I would like stackup to keep retrying for a very long time, with exponential backoff, so that it gracefully handles the case where many deployments are happening at once.

Related to #55 .