add support for ${prop} style replacement to attributes in the web subsystem

It would be beneficial to allow ${property} style substitution on many if not all attributes of the web subsystem.
This was present in JBoss 5.x, so customers are in effect loosing functionality when upgrading.

Right now we are specifically looking at getting support for the ssl tag of a connector:

attribute: key-alias

But it might be easier & more consistent to simply add support for this to all attributes in one go, thereby avoiding to have to revisit this later for more attributes.