Added an example for configuring the variables for the OpenID, that might fails because the variables are not defined for the playbook parsing, but Synapse's own config file parsing.

Adding more presets in the future would be nice.

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.

Fixes #562 (Github Issue)