auto generate gate jobs

Today the list of what jobs are run as part of each gate is hand defined in the openstack-infra/config repo in modules/openstack_project/files/zuul/layout.yaml. As the number of different jobs in the gate grows we want to:

* Not run jobs that don't test the project at hand so we can optimize the usage of our finite number of test nodes
* Make sure we don't have any holes in our gate that can cause the gate to be wedged (ex: job x uses cinder but we don't gate cinder on job x).

One way to implement this is to use the service list: so if a job passes with cinder disabled then cinder isn't a dependency and we don't need to gate cinder on this job.

Note: This BP is looking for an assignee