tech debt: links ref access via core

[viniarck]

When topology links move to kytos core kytos-ng/kytos#92, this NApps shouldn't be directly accessing self.controller.napps[("kytos", "topology")]:

https://github.com/kytos-ng/maintenance/blob/master/main.py#L232

More NApp-to-NApp access will be allowed via NApp instance ref but that will still be specified.