[FIXED JENKINS-15576] Label's Load Statistics queue length based on label assigned to job rather than to item in build queue.

This fix allows the Node and Label parameter plugin to be used in conjunction with Cloud implementations. The plugin dynamically assigns labels to builds, but currently the LoadStatisticsUpdater bases the per label queue length on the label assigned to the parent job configuration, rather than to the build in the queue itself.

When the Node and Label parameter plugin is not used the label assigned to the build matches the parent job, therefore in that case the current behaviour is unchanged by this fix.

LabelLoadStatisticsQueueLengthTest has 2 tests - one which tests the new behaviour when a project is scheduled with a LabelAssignmentAction, and another which tests the old behaviour is maintained when no label is assigned to a build.