Update bbsim grpc-gateway to output swagger documentation in a cleaner format

The sphinx documentation generator's openapi parser complains when runing various targets that build the docs:

bbsim/docs/source/api.rst:None: (ERROR/3) HTTP status code must be an integer (e.g. '200') or start with an integer (e.g. '200 OK'); <#text: 'default'> is invalid (edited)

This is an OpenAPI schema violation: https://github.com/OAI/OpenAPI-Specification/blob/master/schemas/v3.0/schema.yaml#L566

But the Swagger/OpenAPI site mentions `default` in the Responses section here: https://swagger.io/docs/specification/basic-structure/

This is fixable by updating the grpc-gateway tool that generates the *.swagger.json files to v1.14.0 or later and passing the 'disable_default_errors=true' option in the bbsim makefile : https://github.com/grpc-ecosystem/grpc-gateway/blob/v1.14.0/protoc-gen-swagger/main.go#L31

Update the grpc-gateway supplied by voltha-docker-tools to use the newer version.