PortMod should be sent when an interface is disabled or enabled

[viniarck]

Currently, when an interface is disabled even tough the enabled state will be set as false, a PortMod OpenFlow message isn't being sent, as a result it's possible to have an interface enabled: False, and active: True, since it won't actually administratively disable the interface on the switch.

When implementing this feature, the PortConfig config should correctly use the OFPPC_PORT_DOWN flag, and the advertise capability shouldn't be impacted. This also should be tested with NoviFlow switches to make sure it'll work as intended and no side effect is observed, simulating a disable/enable should bring up a link correctly. Also, an end-to-end test should be added to make sure we have it covered for OvS OF 1.3.

An existing workaround, even though it won't send the actual PortMod message, but if an operator were also to shut down via another management plane that would have an equivalent effect, but it's not that practical to keep doing this.