Avoid sending a request if the switch just reconnected

[viniarck]

Quoting @italovalcy to capture this requirement on this comment:

@viniarck another enhancement opportunity is: we should only request a flow_mod add when it is a new switch (or it was previously disabled before). The reason basically related to the fact the flow_manager should take care of reinstalling the flows once the switch reconnects. Currently, once a switch reconnects a new kytos/topology.switch.enabled event is generated (https://github.com/kytos-ng/topology/blob/master/main.py#L517-L530) and LLDP flows are requested again (even though they probably exists at the switch, if the switch only reconnected).
For the kytos/topology.switch.disabled event (the else branch of the conditional above), then it makes sense to remove the flows.