Undertow subsystem enhancements

-  WFLY-3368 Better reverse proxy configuration
- Properly implement error-pages as filters 
- WFLY-3672 Add configuration to enable PeerNameResolvingHandler
- WFLY-3697 Custom filters support
The idea was that the path and handlers resources are always defined, otherwise it may take either one or two add operations to create a handler or a filter, as you would first need to create the parent resource.