DynamoDB, Git and Autoscaling Improvements

DynamoDB Indexes Enhancements

For a majority of users, the decreased provisioned throughput for build and release DynamoDB tables will result in a monthly savings!

Thanks @frsechet for the improvement!
For more info see PR #736: changed provisioned throughput for DynamoDB indexes.

Git Submodules Checkout

This enhancement allows convox build to pull/update a repo's submodules, if any.

Thanks @mfine for the improvement!
For more information see PR #732: Update submodules on git checkout

Autoscaling Improvements

PR #737 made some improvements for scaling a rack. With the rack Autoscale parameter enabled,
there is no need to use the --count option to increase the instance count. A Rack can scale its own instance count based on the needs of the containers it provisions.