Provide XML schema for credential-reference

With https://issues.jboss.org/browse/EAP7-538, all passwords references in the subsystems must support credential references.

The model attribute is provided by wildlfy-controller module (including its XML parser and marshaller) but the corresponding XML element has been added directly to the subsystems XML schemas. This will become a maintenance issue when/if the credential references has to be updated and all the subsystems schema risk to be out of sync with the actual code reading/writing the credential reference.

We should follow the same design than for threads attributes (provided by the willdfly-threads module): the wildlfy-controller module provides a XML schema for the credential-references with its own versioned namespace and any subsystem that requires it will just refer to it.