Add XSD validation of subsystem's XML

add methods in AbstractSubsystemBaseTest to validate the
subsystem's XML (either from the tests or from the templates
used to build the server configuration) against a given XSD schema

Subsystems need to override getSubsystemXsdPath() and
getSubsystemTemplates() to active the validation tests.

* validate IO susbystem's XML as an example
I've added a commit to the PR to have a proper component upgrade for jboss-metadata-common in wildfly-core and uses its 8.1.2.Final version