[20180808194846] Service Substacks and Mysql Resource

The mysql resource type is now available to be declared inside the convox.yml.

App services have been moved out into nested CloudFormation stacks to prevent reaching a hard limit of 200 resources per stack. Application services will temporarily use double capacity during their next deployment after updating to this release. Racks that use Autoscale=Yes should handle this invisibly, otherwise you will want to make sure you scale up to some extra capacity to deploy.