[INFRA-488] Add a Jenkinsfile for Jenkins 2.0.

Some hacks in here to deal with building as root.

More work to do - currently, we're not including the Acceptance Test
Harness here, but that may change - it takes a really, really long
time. We're still waiting for
https://github.com/jenkinsci/packaging/pull/34 to get merged and are
using a branch for packaging for the time being. And a lot of
packaging tests are failing - see
https://issues.jenkins-ci.org/issues/?filter=15257 for possible
related JIRAs.

Conflicts:
	cli/pom.xml
	core/pom.xml
	plugins/pom.xml
	pom.xml
	test/pom.xml
	war/pom.xml

[FIX JENKINS-9598] 2.0 with plugin wizard

 [FIXED JENKINS-32328] process multiple update-centers for ToolInstallers

As part of the process of getting a [new plugin parent POM](https://github.com/jenkinsci/plugin-pom) decouple from the core project
the plugin module is removed.

Setting XMLUtil DocumentBuilderFactory features to try close XXE vulnerabilities - fixed state for one of the features

Still didn't work without rejecting doctype completely

Also ... unbundling of plugins from Jenkins core.

BuildTriggerTest.downstreamProjectShouldObserveCompletedParent failure

[FIX JENKINS-31753] Fields are aligned at the bottom

[JENKINS-26466] Decouple Node from NodeMonitor

Another fix for the future node_monitors package extraction.

This time, a simple import removal.