Advertise known neuca-images

[ahelsing@bbn.com]

It would be helpful to tools and experimenters if the Ad RSpec listed the Disk Images that are currently listed on the Orca wiki (​https://geni-orca.renci.org/trac/wiki/neuca-images).

As at other aggregates, the assumption should be that many other images are possible, but that these are a few that are known to work - one of which is the default that you will get if you do not specify an image.

That is, each <node> in the Ad RSpec would have multiple sub-elements like this:

      <disk_image description="SStandard Centos 6.3 image" name="http://geni-images.renci.org/images/standard/centos/centos6.3-neuca-v1.0.6.xml" os="Centos" version="6.3"/>