Add Default Base Environment as Prod

[eakmanrq]

When someone creates a new environment we don't know what to compare their changes against to detect if there are changes. We currently just say that everything is added and breaking. This change will introduce a concept of a default base environment that we will compare against when creating an environment for the first time. We will start by just defaulting to this to prod. There is a follow up issue to allow users to custom this value when running plan.