CloudFormation service cast most types into strings

[eduardomourar]

The values passed down from CloudFormation service to the actual resource provider are strings, instead of types specified in schema. The solution at this moment is for each resource provider to handle that, but the support library should be able to cast into types from model.

This is most probably related to aws-cloudformation/cloudformation-cli#435 and aws-cloudformation/cloudformation-cli-python-plugin#80.