.gitignore - ignore certbot keys

[cantoute]

Storing certbot/letsencrypt keys in the repo takes considerable space, specially when having many domains over long time. ...and obviously is a security risk

I have not fully tested yet, but with those rules we should still keep trace of certbot setup, at disaster this should be good enough to re-create all new certificates at once.

There probably is a more elegant way do this, but I think it would be a good idea not to store those keys by default.

Thanks for your great work, this package is very handy and made this world a better place :)