Add metadata to property to AutoScale ServerArgument

Adding metadata as described here http://docs.rackspace.com/cas/api/v1.0/autoscale-devguide/content/Launch_Configuration.html.

This is needed to subscribe auto scale servers to dedicated load balancer pools.
There is an extra space after the `,` here. (Not a big deal, but it's easy to correct at the same time as amending the commit to address the line endings problem.)