Swagger-Models: Make definitions use a sorted Map instead of HashMap

[domdorn]

io.swagger.models.Swagger#definitions is using a HashMap as its implementation. This leads to unstable results when serializing the spec to json.

I'd like to change the implementation to use a TreeMap<String, Model> instead. This makes sure, that the map is always sorted the same way.

I'm happy to provide a pull request.

[ChristianLutz]

Currently this PR has been applied to oas3. It uses an LinkedHashMap instead of a TreeMap. I would also prefer the later.

[hiddewie]

The maven plugin from OpenAPI tools has to sort the output manually now. This is a problem for projects where the OpenAPI specification is committed. Also see openapi-tools/swagger-maven-plugin#49.