Fixes for JENKINS-21350, JENKINS-21351 and JENKINS-21370

This pull request includes fixes for those issues:

1.  JENKINS-21350: Here we get a NullPointerException when checking out a    
     variant project where the original project has been moved. In my opinion 
     this is a major problem and it would be good to have it fixed in one of the 
     next plugin versions.
2. JENKINS-21351: This is just a nice-to-have. A little inconsistency since 
    Groovy expressions can be used in the main SCM config section but not in 
    the checkin post-build action.
3. JENKINS-21370: The addprojectlabel functionality had been removed with 
    1.20. This feature has been recovered and extended with the functionality to 
    recurse subprojects for applying checkpoint labels to subprojects as well.

Best regards
Christian