[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.
`makeDisabled` is specific to `AbstractProject`, and any build whose parent is an `AbstractProject` must itself be an `AbstractBuild`.