Non-pooled state for resource

[mthuurne]

When setting up a new Task Runner, you typically want to force one or more jobs to run on that Task Runner to validate the setup. This can be done with Task Runner selection. But production tasks in the factory could be automatically assigned to the new Task Runner before it's ready. So it would be useful to have a flag that indicates that a Task Runner is only available for explicit selection and should not be picked for any task that doesn't explicitly select it.

While the scenario I describe involves Task Runners, this would apply to other types of resources as well. For example during the migration from one repository to another, it would be useful to run test jobs using the new repository while production jobs keep using the old repository.