The different configurations are now all lower case, for consistent
naming.
`matrix_nginx_proxy_ssl_config` is now called
`matrix_nginx_proxy_ssl_preset`. The different options for "modern",
"intermediate" and "old" are stored in the main.yml file, instead of
being hardcoded in the configuration files. This will improve the
maintainability of the code.
The "custom" preset was removed. Now if one of the variables is set, it
will use it instead of the preset. This will allow to mix and match more
easily, for example using all the intermediate options but only
supporting TLSv1.2. This will also provide better backward
compatibility.
This broke in 63a49bb2dc.
Proxying the OpenID Connect endpoints is now possible,
but needs to be enabled explicitly now.
Supersedes #702 (Github Pull Request).
This patch builds up on the idea from that Pull Request,
but does things in a cleaner way.
