Allow the Admin to restrict certain Agents to be available to certain plans only

Important: this is not the functionality that can be achieved with the Custom Capabilities. We don't want to direct a Plan to be run in an agent, but to make some agents unavailable to some plans.

For instance, having Team A and Team B. We want Team A to run its Plans on Agent A only and Team B to run its Plans on Agent B only. Since there are limited resources, if Agent A is used by Team B, this will upset Team A, as it will have to wait B finishing using the resource.