[JENKINS-23365] Work in a general Job

See https://github.com/jenkinsci/jenkins/pull/1257 for background. Will not build right now because we are waiting for 1.568 to be released in a few days, but filing early in the hope of getting comments.
The <code>instanceof</code> check is quite suspicious. 
I suppose that other <code>Run</code> implementations should have similar functionality if their jobs inherit from <code>AbstractProject</code>