Call read handler on create/update and return empty model on delete - #129

Description of changes:
Updates the handlers.py template file to be more in line with the contract definitions - https://docs.aws.amazon.com/cloudformation-cli/latest/userguide/resource-type-test-contract.html

* This PR adds the call to `read_handler` at the end of create and update, instead of returning a success progress event.
  My understanding is that this is required for stabilization and also ensures the model contains any readOnly attributes

* It also sets the model to None when returning a progress event in the delete handler.

When the delete handler returns SUCCESS, the ProgressEvent object MUST NOT contain a model.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

read handler delegate makes sense on the last invocation as the create/update handler output must be equal to read handler output, however throughout execution we should return progress as it contains information necessary to the callback