OVSDriver: use MAC from getifaddrs if no OVS_VPORT_ATTR_ADDRESS

It looks like OpenVSwitch stopped supporting the OVS_VPORT_ATTR_ADDRESS. But,
IVS seems to depend on that for setting the MAC address of a port and failed.
This caused the controller to assume the port MAC address to be
00:00:00:00:00:00 and use that for the LLDP packet-outs. These LLDPs were
promptly dropped by our Dell switch.
Merge into master from pull request #56:
OVSDriver: use MAC from getifaddrs if no OVS_VPORT_ATTR_ADDRESS (https://github.com/floodlight/ivs/pull/56)