Use general description, summary and operationId in generate openapi2 and postman

[tronto20]

I use both openapi3 and openapi2. But their description, summary, and operationId were generated differently in the documents. (with multiple snippets)

When Openapi3Generator generate openapi3, it determine them smartly.
However, Openapi20Generator and PostmanCollectionGenerator choose them from the first snippet.

Issue #63 is for choose summary and description, and applies in #69.
Issue #140 is for choose operationId, and applies in #141.
I want to apply these issues to Openapi20Generator and PostmanCollectionGenerator