Add API mac learning extension for NVP

This commit adds an API extension for NVP where the
NVP supported mac learning feature can be switched
on/off for a specific port. The attribute can be
True or False or omitted altogether.

Implements blueprint nvp-mac-learning-extension

Change-Id: I9173c7dfe0cf4a9ee7b0605722ce7fa01708f5ba
Merge "Add API mac learning extension for NVP"