Integrate network (neutron) information in the model

As of now, Watcher only provides compute information from its model. The objective would hence be to extend it to also provide network-related data via Neutron.

Objectives:
- Be able to integrate network info at the model build stage
- Be able to consume all the needed network notifications in order to maintain the consistency of the network-related part of the model
- Be able to easily query/retrieve the network information from within a strategy via a clear set of methods