read availability_zone from config if its set in config file. If not set by user or config then raise error

read availability_zone from config if its set in config file. If not set by user or config then raise error since SLapi requires it to be specified.
This addresses https://github.com/softlayer/jumpgate/issues/81
For clarity, thThis could be simplified to:

```python
        datacenter = (lookup(body, 'server', 'availability_zone') or
                      CONF['compute']['default_availability_zone'])
```