Document difference between port.created and interface.created events

[ajoaoff]

Original issue opened by @cmagnobarbosa at kytos#116.

Today Kytos uses two events (port.created and interface.created) when a port on the switch is created, but the difference between these events is not clear.

• https://github.com/kytos/of_core/blob/master/main.py#L462L485

Therefore, additional research is needed.

Additional information from OpenFlow spec:

OpenFlow ports are the network interfaces for passing packets between OpenFlow processing and therest of the network. OpenFlow switches connect logically to each other via their OpenFlow ports. An OpenFlow switch makes a number of OpenFlow ports available for OpenFlow processing.

The set of OpenFlow ports may not be identical to the set of network interfaces provided by the switch hardware,some network interfaces may be disabled for OpenFlow, and the OpenFlow switch may define additional OpenFlow