Define and model 'Endpoints'

[kevgliss]

Lemur should have some idea where certificates are deployed. This will give us increased visibility into the impact of a given certificate and help us build more advanced deployment strategies on top of this information.

Endpoint should be generally abstract, consisting of an IP/DNS and or ELB Name. This combined with the port, should be created a uniquely identifiable endpoint from which an TLS certificate could be served.

Once defined the endpoint model could be defined manually or automatically via a discovery process.