Set the MAC address for ensure-bridge bridges

We're seeing the bridge sometimes not glue to the right port address.
The defined behaviour is to dynamically change the MAC based on
attached ports and as we need DHCP to work, we should set the MAC to
the MAC that Neutron expects.

Co-Authored-By: Michael Kerrin <michael.kerrin@hp.com>
Closes-bug: #1329238
Change-Id: I87302d468784a9c9d1703f485031016f87d57873
Merge "Set the MAC address for ensure-bridge bridges"