sshd_config: Simplify type and provider with key as namevar

Thanks to @crayfishx I just realized I had a bad understanding of `namevar` up to now. This PR simplifies and `sshd_config` type and provider without changing anything of the interface.

There is no need to manage `name` manually, it's automatic in types. By setting `key` as namevar, the overriding process is also automatic, and `key` is set to `name` whenever it is not specified.