CORS headers needed on aws-python-rest-api-with-dynamodb example

[mikelmaron]

In https://github.com/serverless/examples/tree/master/aws-python-rest-api-with-dynamodb, cors: true is set in configuration of each endpoint, but that only covers pre-flight CORS requests. Need to add headers in the response object as well, per https://www.serverless.com/blog/cors-api-gateway-survival-guide