Add alias for openstack availablilty zone server attribute

The availability zone attribute in server API messages is actually
OS-EXT-AZ:availability_zone.  Adding the alias will correctly map the
availability zone values into fog responses.
Merge pull request #2195 from blomquisg/openstack-availability-zone

Add alias for openstack availablilty zone server attribute
[vcloud_director] More mocking (1.8.7 compliant)