add gateway on router for L3 API

[feyman]

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I found the python-openstacksdk has a "add_gateway_to_router" method which is finally mapped to /v2/routers/{router_id}/add_gateway_router in the RESTful api with `PUT` method, however, in the official doc, there are only /v2.0/routers/{router_id}/add_router_interface API for adding interface on router. As far as I can see, it's necessary to introduce add_gateway_router and remove_gateway_router API

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-----------------------------------
Release: 1.7.1.dev53 on 'Thu Jul 6 17:54:16 2017, commit 4e100d9'
SHA:
Source: Can't derive source file URL
URL: https://developer.openstack.org/api-ref/networking/v2/index.html

[Rodolfo Alonso (rodolfo-alonso-hernandez)]

Bug closed due to lack of activity, please feel free to reopen if needed.