[DO NOT MERGE][FIX JENKINS-25449] Form validation for label expression

[JENKINS-25449](https://issues.jenkins-ci.org/browse/JENKINS-25449)
![matrix-expr-validate](https://cloud.githubusercontent.com/assets/206841/4919049/c796556a-64f6-11e4-9d3d-d07be66548db.png)

Let's expose the method explicitly: https://github.com/jenkinsci/jenkins/pull/1456