Run a proper linter on the entire template

I'm wasting a lot of time debugging errors in CloudFormation templates. They take minutes to process when I debug with the `push -y` method.

What `amplify push -y` should do is run an extremely accurate linter. The linter should be run over every template, or simply process the root + nested as a single file.

Integrating `cfn-lint` would be a good start.