Add knobs for setting MAC learning on ports

If the Neutron MAC learning API extension is supported, allow user to enable it
for a port. Also display the current MAC learning state in the Ports table and
Port Detail page.

Change-Id: Ia4c4e9a4116527aae8b5488ff8a75eb4113bb8cf
Implements: blueprint mac-learning-knobs
Merge "Add knobs for setting MAC learning on ports"