NIFI-4357 Global improvement of XML unmarshalling

[alopresto]

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[mcgilman]

Will review...

[mcgilman]

Thanks for the PR @alopresto! I've run into a small issue with one of my tests. When I upload a template into my NiFi instance subsequent restarts fail with the following exception:

org.apache.nifi.controller.serialization.FlowSynchronizationException: java.lang.RuntimeException: Could not parse XML as a valid template
	at org.apache.nifi.controller.StandardFlowSynchronizer.sync(StandardFlowSynchronizer.java:436)
	at org.apache.nifi.controller.FlowController.synchronize(FlowController.java:1591)
	at org.apache.nifi.persistence.StandardXMLFlowConfigurationDAO.load(StandardXMLFlowConfigurationDAO.java:84)
	at org.apache.nifi.controller.StandardFlowService.loadFromBytes(StandardFlowService.java:723)
	at org.apache.nifi.controller.StandardFlowService.load(StandardFlowService.java:534)
	at org.apache.nifi.web.contextlistener.ApplicationStartupContextListener.contextInitialized(ApplicationStartupContextListener.java:72)
	at org.eclipse.jetty.server.handler.ContextHandler.callContextInitialized(ContextHandler.java:876)
	at org.eclipse.jetty.servlet.ServletContextHandler.callContextInitialized(ServletContextHandler.java:532)
	at org.eclipse.jetty.server.handler.ContextHandler.startContext(ContextHandler.java:839)
	at org.eclipse.jetty.servlet.ServletContextHandler.startContext(ServletContextHandler.java:344)
	at org.eclipse.jetty.webapp.WebAppContext.startWebapp(WebAppContext.java:1480)
	at org.eclipse.jetty.webapp.WebAppContext.startContext(WebAppContext.java:1442)
	at org.eclipse.jetty.server.handler.ContextHandler.doStart(ContextHandler.java:799)
	at org.eclipse.jetty.servlet.ServletContextHandler.doStart(ServletContextHandler.java:261)
	at org.eclipse.jetty.webapp.WebAppContext.doStart(WebAppContext.java:540)
	at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
	at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:131)
	at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:113)
	at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:113)
	at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
	at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:131)
	at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:105)
	at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:113)
	at org.eclipse.jetty.server.handler.gzip.GzipHandler.doStart(GzipHandler.java:290)
	at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
	at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:131)
	at org.eclipse.jetty.server.Server.start(Server.java:452)
	at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:105)
	at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:113)
	at org.eclipse.jetty.server.Server.doStart(Server.java:419)
	at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:68)
	at org.apache.nifi.web.server.JettyServer.start(JettyServer.java:707)
	at org.apache.nifi.NiFi.<init>(NiFi.java:160)
	at org.apache.nifi.NiFi.main(NiFi.java:268)
Caused by: java.lang.RuntimeException: Could not parse XML as a valid template
	at org.apache.nifi.controller.TemplateUtils.parseDto(TemplateUtils.java:51)
	at org.apache.nifi.controller.StandardFlowSynchronizer.addProcessGroup(StandardFlowSynchronizer.java:1384)
	at org.apache.nifi.controller.StandardFlowSynchronizer.sync(StandardFlowSynchronizer.java:322)
	... 33 common frames omitted
Caused by: java.lang.UnsupportedOperationException: Cannot create XMLStreamReader or XMLEventReader from a javax.xml.transform.dom.DOMSource
	at com.sun.xml.internal.stream.XMLInputFactoryImpl.jaxpSourcetoXMLInputSource(XMLInputFactoryImpl.java:302)
	at com.sun.xml.internal.stream.XMLInputFactoryImpl.createXMLStreamReader(XMLInputFactoryImpl.java:143)
	at org.apache.nifi.security.xml.XmlUtils.createSafeReader(XmlUtils.java:43)
	at org.apache.nifi.persistence.TemplateDeserializer.deserialize(TemplateDeserializer.java:41)
	at org.apache.nifi.controller.TemplateUtils.parseDto(TemplateUtils.java:49)
	... 35 common frames omitted

[mcgilman]

Thanks @alopresto! I've made some minor adjustments to your PR to address the merge conflicts and the error I reported earlier. This has now been merged to master.