Group endpoints by domain

Currently the Endpoint Manager only takes into account the local name of the endpoint.
The endpoints should be grouped by domain name and associated with an EndpointManager responsible for all endpoints of a given group.

Example:

![mgcp-classes-endpoint-manager](https://cloud.githubusercontent.com/assets/7796608/17780983/47b2f3e6-6565-11e6-8949-3d01e060f6c5.png)
