Add Ubuntu distribution channel

[HLFH]

Ubuntu supports your package certbot-dns-gandi.

I advise https://github.com/obynio/certbot-plugin-gandi/issues/22 to be closed. For these reasons:

• The Ubuntu packaging guide does not recommend snap crafting but deb packaging: https://packaging.ubuntu.com/html/packaging-new-software.html ;
• Ubuntu MOTU Developers maintain a certbot-dns-gandi deb package: https://packages.ubuntu.com/kinetic/python3-certbot-dns-gandi ;
• This package is actively maintained with the latest version available on the latest Ubuntu: https://launchpad.net/ubuntu/+source/python-certbot-dns-gandi/1.4.2-1
• Only Ubuntu Core, a special Ubuntu spin-off for the IoT, is solely using snap packages ;
• The Ubuntu main flavours use both apt & snap and the main Ubuntu packaging policy only gives direct guidance for deb/apt packaging ;
• We are years away for snap replacing apt as default package manager.

If a snap package is built by a third-party maintainer for certbot-dns-gandi, you can accept a PR updating the README file such as:

* PyPI: https://pypi.org/project/certbot-plugin-gandi/
* Archlinux: https://aur.archlinux.org/packages/certbot-dns-gandi-git/
* Debian: https://packages.debian.org/sid/main/python3-certbot-dns-gandi
* Ubuntu: https://packages.ubuntu.com/kinetic/python3-certbot-dns-gandi
* Ubuntu (snap): https://snapcraft.io/certbot-dns-gandi (does not exist yet)

Until a third-party maintainer decides to craft and publish an Ubuntu certbot-dns-gandi snap package, the official package for Ubuntu should be the only recommended way.

[obynio]

Oh well, I'm learning that there is a Ubuntu Let's Encrypt Team :D

I'm not a big fan of snapd so I doubt I will work on an official package when a deb package already exists, but if someone is willing to maintain one feel free to open an issue. Anyway let's merge this PR, seems like it's a fine addition.