XML Validation of subsystems

add JBossEntityResolver to resolve imported schema

* XML validation of remoting and threads subsystem
* logging XML can not be validated due to the use of @@ placeholders (the XML validation does not take into account any supplement's replacement)

The pending commits for the wildfly project are at https://github.com/jmesnil/wildfly/commits/xsd_validation_subsystems. I'll open a PR for wildfly with them once wildfly-core 1.0.0.ALPHA15 is released (as some subsystem requires to resolve imported schema)  to be validated).
I'm not in favour of modifying the class hierarchy.

If a subsystem produces some XML, it must provide a XML schema and checks that the produced XML is valid (the consumed XML can be invalid though).

This part of the contract should be verified by AbstractSubsystemBaseTest. 
The framework just needs a way to opt out from this for special uses of the AbstractSubsystemBaseTest (e.g. in the logging subsystem) and this can be done by returning an empty list from getSubsystemXsdUrls().