The value of `*.ssl.client_authentication` is `required` for
everything except `xpack.security.http.ssl.client_authentication`, for
which is it `none`.
The doc template for this setting was configured to have a default
value, and allow an override. However, the default was set to `none`
when it should have been `required`.
The override for `http` was correctly set to `none` (but that didn't
really do anything, since that was the same as the default).
This commit changes the default to `required`, which matches the code
(see `XPackSettings.CLIENT_AUTH_DEFAULT`), and leaves the override for
http as `none` (see `XPackSettings.HTTP_CLIENT_AUTH_DEFAULT`).
7ce9398fcf