Notifications for interface attach/detach

While nova sends notifications on volume attachment/detachment, it does not do the same for interface attach/detach. Neutron's notifications don't give enough information to determine when a port is detached from an instance because they contain only the current state of the port (and thus device_id is null).

My use case is for indexing current state for Searchlight, but I think there's a case purely for consistency in that these actions affect a server's state similar to volume operations and so should send a notification.

Referring to https://bugs.launchpad.net/nova/+bug/1565028