Reserve task slots for interactive jobs

[soichih]

We currently have only 6 slots for sca-product-raw service for data staging. When a batch process gets executed, it takes up all available slots which prevents users from trying to visualize, or submit tasks manually via web UI.

Although I've already implemented "nice" capability to make batch submitted tasks to have lower handling priority than interactively submitted ones, we could improve UX by reserving a few slots for resource with limited amount of max task.