cloudformation: use template for Atlas MongoDB deployment

[tschaume]

The Atlas MongoDB deployment and its peering connection from the AWS VPC is currently set up manually every time a new pipeline and thus a new collection of stacks is required. This can happen, for instance, if development stacks are needed, a new region is explored, or new infrastructure components are being tested.

The solution is to use MongoDB's Atlas Resource Providers in the Cloudformation templates directly. This would also set up peering connections automatically.

This change will likely result in a brand new Atlas MongoDB deployment which will implicitly use the new Private Connection Strings. The environment variable for the API containers will have to be updated accordingly.