Add support for JDK axis

Axis that selects available JDKs. JDK axis has the same XML
structure as other axes; only the element name changes.

The matrix project support was split out of core Jenkins into the
'Matrix Project Plugin', so update the docstring to mention that
this plugin is required.

Tested on Jenkins ver. 1.565.2 with Matrix Project Plugin 1.4.

Change-Id: I6b9d243810833bd3d31535439cc48768db5a6598

tests/general/fixtures/matrix-axis004.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="utf-8"?>
+<matrix-project>
+  <executionStrategy class="hudson.matrix.DefaultMatrixExecutionStrategyImpl">
+    <runSequentially>false</runSequentially>
+  </executionStrategy>
+  <combinationFilter/>
+  <axes>
+    <hudson.matrix.JDKAxis>
+      <name>JDK</name>
+      <values>
+        <string>openjdk-6-jdk</string>
+        <string>openjdk-7-jdk</string>
+      </values>
+    </hudson.matrix.JDKAxis>
+  </axes>
+  <actions/>
+  <keepDependencies>false</keepDependencies>
+  <blockBuildWhenDownstreamBuilding>false</blockBuildWhenDownstreamBuilding>
+  <blockBuildWhenUpstreamBuilding>false</blockBuildWhenUpstreamBuilding>
+  <concurrentBuild>false</concurrentBuild>
+  <canRoam>true</canRoam>
+</matrix-project>

tests/general/fixtures/matrix-axis004.yaml

@@ -0,0 +1,8 @@
+name: matrix-test004
+project-type: matrix
+axes:
+    - axis:
+        type: jdk
+        values:
+            - openjdk-6-jdk
+            - openjdk-7-jdk