Add forall_vanir function

allows for neat stuff like...
    to add a tag to every vanir-tracked project:
         forall_vanir git tag nukeistheman
    to add a NEW branch called jb43 for every vanir-tracked project to gerrit:
         forall_vanir 'git push ssh://<username>@vaniraosp.goo.im:29419/$REPO_PROJECT HEAD:refs/heads/jb43'

Change-Id: I9e55b8d982b4682d73fedd7eb5b462ce2caf95b2