Replace mac_address with port_mac in Port class

This patch fixes the inconsistent attribute name which is different from
the name specified in midonet-cli file introduced by d49a7ea2967.

This fixes MN-1911.

Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Merge pull request #178 from tfukushima/mn-1911-master

Replace mac_address with port_mac in Port class