Support for optional service operations

[andrestc]

Currently, every service has to implement all the endpoints. For some services, bind/unbinding app and bind/unbinding unit do not make sense.

Service owners should be able to configure on the Service configuration file if they want to disable such operations.

This will make easier for service owners to implement more services (less boilerplate) and also may save lots of requests (especially for apps with many units and bound to multiple services).