[FIXED JENKINS-7291] Flyweight jobs and zero executors

Seems to fix [JENKINS-7291](https://issues.jenkins-ci.org/browse/JENKINS-7291). Probably needs a test using `DummyCloudImpl`, but filing now for feedback.

Also it is not clear if there are security implications of running a task on master when the admin has asked that this not happen. For `MatrixProject` it is fine. What about Groovy scripting and the like? I guess in these cases we assume that if you can run the script you can do so on master too. In other words, we may need to document that a `FlyweightTask` must be considered trusted.