[FIXED JENKINS-1613] Ability to build on configurable subset of matrix

configurations

This change introduces the necessary extension point. The implementation
of this will be in a plugin.

changelog.html

@@ -64,6 +64,9 @@ Upcoming changes</a>
   <li class=bug>
     Fixed a <tt>ClassCastException</tt> caused by multiple loading of the same class in different classloaders.
     (<a href="http://issues.jenkins-ci.org/browse/JENKINS-9017">issue 9017</a>)
+  <li class=rfe>
+    Support rebuilding a subset of matrix configurations
+    (<a href="http://issues.jenkins-ci.org/browse/JENKINS-1613">issue 1613</a>)
   <li class=rfe>
     Gracefully handle old slave.jar to avoid <tt>AbstractMethodError</tt>
     (<a href="https://groups.google.com/d/topic/jenkinsci-dev/KqFw4nfiQdE/discussion">thread</a>)