fix nova list service issue related to mac addr

the latest openstack add a extension attribute to server model, which
caused the NovaListServer example failed. see
https://blueprints.launchpad.net/nova/+spec/os-ext-ips-mac-api-extension
for details.
Merge pull request #40 from xiaohanyu/master

fix OS-EXT-IPS-MAC:mac_addr issue