Explanation
Creating multiple cloudformation templates is an example of using nested stacks. The advantage of using nested stacks is given below as per the AWS documentation As your infrastructure grows, common patterns can emerge in which you declare the same components in each of your templates. You can separate out these common components and create dedicated templates for them. That way, you can mix and match different templates but use nested stacks to create a single, unified stack. Nested stacks are stacks that create other stacks. To create nested stacks, use the AWS::CloudFormation::Stackresource in your template to reference other templates.
For more information on Cloudformation best practices, please refer to the below link:
* http://docs.aws.amazon.com/AWSCIoudFormation/latest/UserGuide/best-practices.html

In Ansible's variable precedence, the highest precedence is the extra vars option on the command line.
Reference:
http://docs.ansible.com/ansible/playbooks_variables.html#variable-precedence-where-should-i- put-a-variable

Since you are not using VPC endpoints, outbound requests for the code sitting in S3 are routed though
the NAT for the VPC's private subnets. If this networking fails, runtime bootstrapping through code
download will fail due to network unavailability and lack of access to the Internet, and thus Amazon S3.
Reference: http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_NAT_Instance.html