Startup fails when XSD validation cannot access Internet

After a recent change allowing splitting up the broker configuration across multiple files, starting Artemis fails when the server running broker has no Internet access. xml:specialAttrs references in artemis-configuration.xsd fail because the schema location http://www.w3.org/2005/08/xml.xsd is inaccessible. To reproduce, pull the latest master for 2.5.0-SNAPSHOT, block www.w3.org and try to run a broker. Including the schema in the build would resolve this issue.