Issue 1766 Modified mustache files for Go to support nullable in the … (#1869)

* Issue 1766 Modified mustache files for Go to support nullable in the spec v3.0+; Updated model files running .sh scripts for Go.

* Add "nullable" to fake yaml

* Add sample script for OAS3

* Fix output folder (openapi3)

* Run bin/openapi3/go-petstore.sh

* Update samples

* Update jaxrs-jersey

* Update python and php samples

* Add bin/openapi3/go-gin-petstore-server.sh

* Run bin/openapi3/go-gin-petstore-server.sh

* Update bin/openapi3/go-petstore-server.sh to generate "nullable" samples

* Run bin/openapi3/go-petstore-server.sh

* Fix duplicated `import`

samples/server/openapi3/petstore/python-flask-python2/README.md

@@ -0,0 +1,49 @@
+# OpenAPI generated server
+
+## Overview
+This server was generated by the [OpenAPI Generator](https://openapi-generator.tech) project. By using the
+[OpenAPI-Spec](https://openapis.org) from a remote server, you can easily generate a server stub.  This
+is an example of building a OpenAPI-enabled Flask server.
+
+This example uses the [Connexion](https://github.com/zalando/connexion) library on top of Flask.
+
+## Requirements
+Python 2.7+
+
+## Usage
+To run the server, please execute the following from the root directory:
+
+```
+pip install -r requirements.txt
+python -m openapi_server
+```
+
+and open your browser to here:
+
+```
+http://localhost:8080/v2/ui/
+```
+
+Your OpenAPI definition lives here:
+
+```
+http://localhost:8080/v2/openapi.json
+```
+
+To launch the integration tests, use tox:
+```
+sudo pip install tox
+tox
+```
+
+## Running with Docker
+
+To run the server on a Docker container, please execute the following from the root directory:
+
+```bash
+# building the image
+docker build -t openapi_server .
+
+# starting up a container
+docker run -p 8080:8080 openapi_server
+```