[LOG4J2-341] Enable XInclude for XML configurations. Start work on getting XML configs using XInclude to be validated. Refactor the creation of the document builder in a method; refactor enablement of XInclude in a method. Tests must process the XML document to allow XInclude to kick in before validation is performed.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1514013 13f79535-47bb-0310-9956-ffa450edef68