Auto-generated cross project test job

Would be nice to have a job that can test cross projects. This is similar to ODL's existing "integration" job type except downstream projects that see an upstream patch getting merged should be able to automatically respond and run their own jobs against the artifacts of the upstream job.

This job should provide some way to report back to Gerrit would be a bonus too.

Also a trigger so projects can leave comment and test their patch to downstream too.