[follow-up] patched to_dict method to use prop_name from attribute_map (python-flask, python-blueplanet)

Created by: bukowa

This is based on pull request https://github.com/OpenAPITools/openapi-generator/pull/4587 made by @Shogoki as a fix to https://github.com/OpenAPITools/openapi-generator/issues/4586 also https://github.com/kubernetes-client/python/issues/1228 https://github.com/kubernetes-client/python/issues/1215

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@spacether (2020/05)

1. I think that using attr_map is more obvious than using json_keys - please comment.
2. Some tests are not passing with strange behaviour please take a look ( https://travis-ci.org/github/bukforks/openapi-generator/builds/760203683#L4574 )
3. I think tests regarding chained attr_map in to_dict method should be added?
4. Tests are included only in the python-flask samples for now.