Topology app keeps a local ovs port cache because it does not want all
port updates to take effect. In particular, events with missing data
(ofport, type). Moving this logic to vswitch_impl because it makes a lot
of sense to hold off those events even for apps that consume OvsPorts.
Partial-Bug: #1737339
Change-Id: I0bc8053d6fb3ed277028cf6bce9c6a0269e601af