Make scheme default to null
Make it possible to configure SSL certificate forwarding over HTTP
Merge pull request #5193 from ctomc/cert-forwarding

Make it possible to configure SSL certificate forwarding over HTTP
[WFLY-2171] Use deployment unit processors to register mod_cluster metrics
[WFLY-2090] Rework metrics to be HttpHandlers instead of ThreadSetupActions since they can run multiple times and cause ISE
[WFLY-2171] Introduce UndertowAttachments class to shelter homeless AttachmentKeys used for mod_cluster integration