Update azure ohai plugin to use SharedConfig.xml on linux for node attributes.

Instances is an array -- this grabs the ip from the first element, but I'm not sure we can assume the first element will contain the address for THIS instance. The array contains all the instances in the deployment - I'm guessing its probably in order or creation, rather than having us always be first.

We should correlate this with the id given in Incarnation.  Maybe fix up this code:

```ruby
@instance_id = @shared_config.elements["SharedConfig/Incarnation"].attributes["instance"]
@shared_config.elements.each("SharedConfig/Instances/Instance") do |instance| 
  if instance.attributes["id"] == @instance_id
    @private_ip = instance.attributes["address"]
  end
end
```