CI Integration test misses testing the controller in agent mode

[huikang]

Current CI test using dnet seems not enable the agent mode. After looking at the dnet code, I do not think the current dnet implementation supports forming a gossip cluster.
To start libnetwork controller in its agent mode, it requires if keysAvailable && !c.isDistributedControl() [1]. The keys are generated by the event EventNetworkKeysAvailable.

Since dnet does not issue such event, it prevents the dnet controller enters its agent mode.
Is my understanding correct? Thanks.

[1] https://github.com/docker/libnetwork/blob/73f58e1087cb31968065749115f4a948e6659763/controller.go#L313-L327