Don't return an error if the plugin is unknown

[dcbw]

We need to silently accept unknown plugins in origin, so if we don't know about it, don't return an error that Origin will exit on.

[pravisankar]

If there is a typo in the network plugin name, this will hide the issue and returns nil. Instead I prefer the caller/origin to do something like: if nodeConfig.NetworkConfig.NetworkPluginName != "" { controller, endpointFilter, err := factory.NewPlugin(...) ... }

[dcbw]

I know, but the problem is that we need to support existing OpenShift plugins that are not openshift-sdn. Like Contrail... Those will specify a network plugin name that we don't match in openshift-sdn, but it will still get carried through to Kubernetes and be used there.

[danwinship]

lgtm. @pravisankar we didn't log anything before in that case either, so this is consistent with the old code