Enhancements to availability zones api - JCLOUDS-582

https://issues.apache.org/jira/browse/JCLOUDS-582
No assumptions about number of availability zones, for instance HP Cloud has az1, az2, and az3.  The getLast() is used everywhere in openstack extensions live tests so this is just keeping it consistent. I saw a code comment somewhere that Adrian was using this convention to avoid using a default AZ which is usually the first I suppose.