Topology don't generate event of link_up when a link is enabled

[italovalcy]

Hi,

When a link is enabled by the administrator, the topology napp does not generate a link_up event (or even any other event), so other napps can take actions. This impacts, for instance, mef_eline as described at kytos/mef_eline#230: when we create an EVC with dynamic path and start the topology with everything disable, the EVC will never gets activated even if the admin enables all links later on.