Northbound API validation

Currently it is very easy to crash the controller by simply not providing a needed field via a NBI request. E.g. create a new entity and don't provide some configuration field.

We should propably add a gRPC middleware that will check if all defined fields in the proto definition are actually there and reject the request if something is missing.

https://github.com/grpc-ecosystem/go-grpc-middleware/tree/master/validator + https://github.com/mwitkow/go-proto-validators