Simplify cluster controls

When a cluster is created, a special service is instantiated (an IPluginControl) where all services in the cluster are registered. This is used to identify services in extension points that exist outside of the cluster.

If such functionality is required, it is simpler to create a separate extension point for the cluster and reregister any services that register there. This would be done by the same plugin that receives the reference to the IPluginControl.

The IPluginControl should still be registered as a service, allowing plugins to register additional interfaces e.g. for lifecycle management.