Passing 'null' (None) as the mac address in a port update
request causes the port's mac address to be re-generated
using the base MAC address Neutron uses for VIFs.
This change implementes a temporary lib api definition
with a new converter that will generate valid mac if the
data provided is None.
APIImpact: Port mac_addr regenerated if None passed on update.
Closes-Bug: #1768690
Change-Id: I7d04beea4810718c3b745de8ea97897b1323267e
8f3a066b20